Consult with your organization about the appropriate locations for your patches. Most schools have specifications for exactly where the patches should be worn. Most also ban unapproved patches.
Measure to find the correct location for the patch. Pin it to the gi using two or three pins.
Put on your gi and your belt. Be careful not to stick yourself with the pins or knock the patch loose. Check the patch in the mirror or have your assistant check it for you. Make sure the patch is straight, shifting it slightly if necessary. Add a couple of more pins to make sure the patch doesn't move while stitching.
Thread the needle with about 2 ft. of thread. Knot one end of the thread. Leave an ample amount of the unknotted end of the thread hanging from the other side of the needle,
Insert the needle from the inside of the gi at the edge of the patch. Pull all of the thread to the outside of the gi, checking to make sure the knot is holding on the inside. Catch a small amount, about 1/16 in., of the underneath side of the edge of the patch with your needle. Pull the thread through. Insert the needle back into the gi just under your stitch on the edge of the patch. Insert the needle from the inside of the gi about 1/8 to 1/4 in. from the first stitch. Repeat the process until you have gone all the way around the outside edge of the patch.
Remove all the pins.