This is a guide to using the Twitch launcher to make modpacks and some basic mods for ones that are looking to get into modded Minecraft.
A bit of background on the Twitch Launcher before we begin.
Originally, one of the biggest modded launchers was Feed the Beast, abbreviated as FTB. The FTB launcher was merged with the Curse Voice client. Later, Twitch bought Curse and merged their desktop application with Curse's voice client, bringing us to the Twitch Launcher.
Step 1: Download the Twitch Launcher
The Twitch launcher can be downloaded here: https://app.twitch.tv/download
When you've downloaded it, run it and sign in with either Twitch or Curse. If you don't have either of those accounts, there is a link to create an account on the sign in page.
When you've signed in, click on the Mods tab up at the top and install the Minecraft plugin. This will allow you to make and download modpacks with ease.
Step 2: The Minecraft Mods Screen
And now you see all of you modpacks! Hm? You don't see anything there? Well, that's because you haven't made or downloaded any! Let's go make one now.
Step 3: Setting Up a Modded Profile
Up at the top there's a button that says "Create Custom Profile". When you click on it, the "Create Profile" screen will pop up. Here you'll name it and pick the Minecraft version you want to play with. Name it whatever you want, then pick your Minecraft version. The modloader should just be left as it is. Click Okay when you're ready to move on.
Step 4: Your Modpack
Your modpack should now appear in your Mods screen. Try running it and then come back here for the next step.
Step 5: Adding Mods
Hm? There's no mods, you say? Well, that's because we haven't added any! If you click on your modpack, it should look like this. Click on "Get Mods".
Step 6: Adding Mods
From here you can search most mods and add them to your modpack by rolling over it and clicking install. The reason I say "most mods" is because mods that aren't hosted on Curseforge probably won't appear here. I'll cover those mods at the end of this Instuctable.
Step 7: Run Your Game
And that's pretty much it! On the next step I have some recommended starter mods for those that are new to the modded scene.
Step 8: Recommended Starter Mods
JourneyMap: I personally don't like playing open-world type games without some form of easy navigation. JourneyMap adds a very simple minimap and waypoint system to the game so you can easily keep track of where you are and where other points of interest are.
Iron Chests: For those that just don't have enough storage. Iron Chests adds chests that can be upgraded with metals, from iron all the way to obsidian.
Inventory Tweaks: This is more of a personal favorite, but this makes organization so much easier. A simple middle click will sort the inventory you are currently hovering over, while pressing R will sort your player inventory.
Just Enough Items (JEI): This is easily one of the most used mods for 1.8 and onwards. It shows every item that is in the game, how to make it, and what it's used in. It's also useful for mapmakers that use specific spawners, because it allows the player to have any item by enabling "cheat mode".
Waila (What Am I Looking At): Very useful mod for those that want to know exactly what they are looking at. If you can hit the block or mob, Waila will show a small window at the top of the screen with the block or mob's name. If it's a mob, it will also show the mob's health. There are also settings to show metadata in case you want to know the exact item ID.
Backpacks: How many times have you gone on a deep mining run, only to find out you had run out of inventory space? This mod adds large, small, and workbench backpacks for all your portable storage needs.
Biomes O' Plenty: Vanilla biomes getting boring? This mod add many new biomes for you to explore, adding many new items with it as well.
Dense Ores: Not finding enough resources, even on deep mining runs? Dense Ores adds "dense" variants of ores that drop multiple ores or resources that the normal ore would drop, i.e, Dense Iron Ore drops 3 iron ore.
Step 9: Common Issues
What if the mod(s) I want aren't on the Twitch Launcher?
This part can get a bit technical. Find the mod you want, preferably through its website, the Minecraft Forums, or PlanetMinecraft. Download the version that goes with the version of Minecraft you want to play with. In the Twitch Launcher, right-click the profile you want to add the mod to and click "Open Folder". You should see a bunch of folders and several files. One of those folders will be labelled "mods". Drop the mod from your downloads folder into the mods folder and it should add it.
I got an error when I tried loading the game!
This probably means there's an item from one mod that has the same ID as another item from a different mod. Unfortunately, this means these mods can't be used together without changing the code. Luckily, this mainly applies to older versions of Minecraft (1.6.4 and earlier), so you shouldn't have to worry too much about this.
I got an error when I tried loading a world after I removed a mod!
This just means that whatever was in the world from that mod will be removed. Some things may require a block update, which can be done simply by right-clicking the block.
Step 10: Personal and Community Favorites
I tend to play with a certain core set of mods. All of the recommended starter mods are usually in there, along with several others.
Tinker's Construct: You want a pickaxe that has more durability than a diamond pickaxe? You can make it. You want a pickaxe that can mine out a larger area? We've got hammers. You want a weapon that's not simply a sword or axe? Maybe a ranged weapon that's not a bow? There are so many customization options for tools in Tinker's Construct that the possibilities are quite a few. Easily one of my favorite mods.
Thaumcraft: Do you like a bit of magic and a little extra danger in your world? Thaumcraft adds a load of magical items to the world, and some new mobs. Many useful magical items can be made through the craft of Thaumaturgy with this mod. It also has compatibility with Tinker's Construct using an add-on mod called Extra TiC. Note: This mod is only available up to Minecraft 1.10.2 and is unavailable for Minecraft 1.9
Ars Magica 2: Maybe thamaturgy isn't your thing, or maybe you want to expand your magical knowledge. Ars Magica 2 adds lots of magical items and bosses, and rituals to summon them.
Thermal Expansion and Thermal Dynamics: Do you like machines? Do you like a simple energy system? Do you like new tools and ways to organize things? Thermal Expansion adds machines, tools, and resources, while Thermal Dynamics adds a way to move things around automatically. It also has integrated compatibility with Tinker's Construct, so you can make stuff with TE metals!
Applied Energistics 2 (AE2): This mod adds loads of storage and machines. Ever wanted to store a structure in an item? No? Now you do. AE2 is based around automation, with lots of opportunities for streamlining your production.
Blood Magic: Using life force to fuel dark magic, this mod adds ways to turn other mobs (or your own health) into power for your magical items.
Botania: For those that would rather not use the life force of mobs, there's Botania, a plant-based magic mod. It's really a machines mod disguised as magic, as this mod has plenty of potential for automation.
Draconic Evolution: Ever wanted to have massive energy storage and extremely powerful weapons and armor? Draconic Evolution adds this, though they're not easy to get.
Ender IO: More machines! This time based around Ender Pearls and rarer resources.
Ender Storage (1.8+): Vanilla Ender Chest just not cutting it? How about one that you can color code? Ender Storage adds tanks, chests, and pouches that can be color coded to give you loads of storage, with a total of 4096 combinations!
Extra Utilities: Extra Utilities adds many useful tools, such as the Builder's Wand, which allows the user to build entire walls with a few clicks. It also adds ways to sort chests, detect block updates, color bricks, and compress your cobblestone.
Hardcore Ender Expansion: Ever felt like the End wasn't "end-y" enough? This mods massively expands the End, adding in multiple boss fights, many new mobs, and lots of new blocks and resources.
Iguana's Tinker Tweaks (1.7.10 only): Iguana's Tinker Tweaks adds some changes to the Tinker's tool system, namely replacing parts of a tool, leveling your tools so they get better as you use them, and many more things. The Tinker's Construct mod has been taken over by Iguana, so for newer versions Tinker's Tool Leveling is the equivalent.
Project Red: Are you a hardcore redstone user? If so, then this mod is for you! Project Red adds lots of functionality to redstone, making it easier to build the contraptions of your dreams. It comes in several modules (Compat, Fabrication, Integration, Lighting, Mechanical, and World) so you can choose what you want to use.
IndustrialCraft 2: One of the oldest mods on the modding scene, IC2 is a mod about automation through the use of machines such as the Quarry, Macerator, and Electric Furnace. IC2 uses its own power system, EU, and requires an add-on mod to convert it to RF, which is what most other mods use nowadays.
The world of mods is quite large, and if you don't see it on the Twitch Launcher, you can probably find it on the internet somewhere.