First, take your time. Line up and sew only a couple of inches then stop, check positioning of the fabric and sew a couple more. As much as I hate to do it, I always pin every thing in place. You can use the side of your presser foot as a guide. My machine has a dial to adjust the size of its zig-zag stitch. I use it to swing the needle from one side of the foot to the other. I leave the machine set on the regular forward stitch but this allows me to adjust the width of my seam and still use the foot for a guide. There are also (or at least should be) reference marks on the plate under the foot that can be used on most seams but a true flat felled seam requires you to open the fabric and covers these marks.

This is all from a man who taught himself to sew. I'm sure there are other methods that those skooled in the assemblige of cloth pieces could share but this is what works for me. Practice on some scraps until you are comfortable and remember that most seams are plenty strong even if they are not so pretty. Don't give up.