There are 2 things that make knitting a blanket a tedious activity. Knitting really long rows--I have had 300 or more stitches on my needle at a time--or sewing little pieces of knitting together to finish a blanket.
Long rows are difficult because you need long needles to hold all the stitches. Long straight needles are cumbersome and awkward. Long circular needles are often hard to find in the right diameter and length. Then again there is the difficulty in finishing a row before you set down your work. Long rows take a long time to finish.
Sewing knit pieces is never my favorite part of the project. I often knit the front and back of a sweater as one piece just to avoid sewing a seam. Seams do offer a bit of shaping stabilization for a garment but I do not like how it changes the stretch of a blanket.
I have knit many blankets over the years and this is my new favorite technique. I developed this method one cold winter day--so cold that schools were closed. I wanted to do something warming. I grabbed some yarn and started knitting with no real idea where it would go.
Step 1: Cast on and First Square
I grabbed a pair of needles--size is not all that important. I used a U.S. size 9 (5.5 mm). I used 2 1/2 skeins of worsted weight yarn but you can use whatever kind/weight of yarn you want. The amount you use will depend on what size blanket you intend to make.
I cast on 20 stitches. I knit 40 rows. I like the thickness and warmth of a blanket that is knit every row. If you want a thinner blanket, you can purl every other row. 20 stitches made a 6 inch square.
On the 40th row, I cast off stitches until I was down to only one stitch.
I have made this blanket several times in different sizes and using different yarns. It looks nice in yarn that varies in color since the color changes show off the changes in stitch direction. The changes in stitch direction also give the blanket a bit of extra stabilization--doesn't stretch out of shape like a blanket which is knit in only one direction. You can use the size needles that you have on hand. It will work with any size.
Step 2: 2nd Square
Using the last remaining stitch, cast on stitches until you have 20 stitches again--19 more stitches. Knit the first 19 stitches.
The last stitch will be knit together with the previous block--this gets really easy once you have done it a few times. Pass the last stitch to the right hand needle. Pick up the loop from the last cast off (from previous square). Put the picked up stitch and the 20th stitch from current block onto the left needle and knit both as one stitch. Turn work and knit all the way to the end of the row.
Turn work and knit the first 19 stitches. Pass the last stitch to the right. Pick up the next cast off stitch. Put both stitches on the left needle. Knit both stitches as one. Turn and knit to the end of the row.
Continue until you have joined to each of the cast off stitches. You don't need to count rows. Just keep working until you use up all the cast off stitches of the previous block.
When you get to the last stitch that needs to be joined to the new block, cast off all but the last stitch of final row ending at the corner where the 2 blocks come together.
Step 3: 3rd Block
With the one remaining stitch on your needle, pick up 19 more stitches from the edge of the 1st block. Pick up the stitches by pulling the working yarn through the loop at the end of the row of knitting.
Once you have these 20 stitches, knit 39 more rows. On the last row, cast off all but the last stitch. Be sure to end with this one last stitch on the outer edge of the blanket.
Step 4: 4th Block--third Diagonal Row
The 4th block begins the 3rd diagonal row of blocks.
Cast on 19 stitches so you again have 20. Knit every row joining the current block to the previous block as you did before. This should look and feel like knitting the 2nd block.
Work back and forth until you use up all the available stitches. Cast off as before and pick up stitches from the next block.
Keep adding one more block to each row until you have reached your desired size.
Step 5: Shorter Diagonal Rows
Once you finish the third diagonal row of blocks, you can knit as many additional rows as you need to achieve the width you want for your blanket. Each row will have one more square than the previous row. This is achieved by casting on 19 stitches.
Once the blanket is the desired width, you need to shorten your diagonal rows. Each additional row will have one less square than the previous row. Arrange for your cast off row to end away from the outside edge of the blanket. After the cast offs, pick up stitches from the previous block.
Step 6: Last Square
The last diagonal row will have only one square block. After you finish knitting it, cast off all stitches and weave in the tail ends of your yarn. Block if necessary.
My sons have slept with hand knit blankets for so long that I thought they could not sleep with a regular blanket. My youngest told me that he could sleep with an ordinary blanket, he just preferred the knit ones. They hug you while you sleep.
If you want a super thick blanket, use 2 strands of yarn. Be prepared for it to weigh a ton.