This Instructable will teach you how to satin stitch a celtic knot (or anything else that will fit the instructions. If you are intimidated by this project, do a google image search on 'feis dress' and weep.
Step 1: Transfer the Image to the Fabric
Start with a simple line drawing/image (no shading). Print it out but try and keep it so that it would fit into a picture frame -unless you are doing a wall hanging- make sure you iron on a fabric stabilizer on the back. I used a light board to transfer the image to the fabric but fabric transfer paper also works well. Either way you need to draw over it with a white colored pencil or a fabric pencil so you can handle it and still have something there to sew.
edit: the stabilizer I used was the cheap Joann's stuff for $.99/yard. I can't find the brand because I think it was made by Joann's but Sulky Fuze and Stitch is pretty close. The fabric is 100% cotton quilting fabric. I chose black with scrolls so that the background wouldn't be too busy and take away focus from the knot.
Step 2: Outlines
Once you have the image on the fabric you need to outline it using the colors that you will eventually be filling it in with. If you change your mind about a color after you've outlined it, make sure your cover it completely when filling it in.
Step 3: Filling in the Outlines
I usually fill in one color completely before moving on. I also changed my mind about all of the colors after the outlining but they were different shades of the same color so it won't show through at all. Also, I sew around and around the line at a 90 degree angle. Do not yank on the embroidery floss, this will cause the fabric to ruffle. Seriously don't. You want the lines of the knot to be even. You are filling in, not gathering.
edit: ok I added another image from ms paint of the top and side views as well as the filling it. When filling in, I go around, from underneath up near the dotted line (the outline) and over to the other side and down. Again and again. The green picture is how its broken up. I start on one part, and about 2 inches away make a line, then I half it and half it again until everything is filled in completely. This way it doesn't seem like as much I have to do.
Step 4: Stabilizer Warning
When I initially ironed on the fabric stabilizer, I folded it up for later use. That was a bad idea because the stabilizer was plasticy and I reironed it over a paper towel which then stuck to it. You can also see the back side, which can look sloppy and the front again as the colors get filled in.
Step 5: Finish Filling and Outline
Completely filled in and now I need to outline it it black, this covers up the uneven edges and makes it pop out. This is also why people ask me if it's done on a machine. Instead of one color at a time, this time I randomly do one section at a time.
edit: the outline:: this is your traditional embroidered line/curve. I've drawn in in ms paint for your viewing pleasure, now pretending that it's all black, I used multiple colors to highlight the actual stitches. A good way to start or improve your technique is to write you name is cursive, very large, and then embroider it. Ok now do your dog's name. Ok now find someone else's name. Keep doing it.
Step 6: The End
Frame or add it to another project. Either way congrats. This is 7 1/2 inches by 7 1/2 inches.
edit: I've added others I done in the same exact manner.