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.
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.