1) Start inside of your hem. Hide your knot in the fold of the hem.
2) Right next to where you started, on the other side, pick up with your needle just a few threads in the fabric.
3) Then go right back down into the fold of the hem. The needle will follow that fold, so the thread stays hidden as you make your seam.
4) Pull your needle out of the top of the fold.
5) Right next to where your needle comes out of the hem, pick up a few threads from the other side.
6) Then go right back down into the fold of the hem.
Repeat until you finish your seam. You'll make up the length under the fold of the hem, and you'll secure it witch those threads on the other side.