This stitch is usually used for hems when the seam should stay unseen.
Step 1: Prepare your Thread
1) Thread your needle. pull the thread through as much as you think you'll need for the length of the seam. A good rule of thumb is double the length, you will have a lot of extra, but that's better than coming up short.
2) Double the thread over at the needle. Cut.
3) Make a knot to secure the two ends.
Step 2: Slip Stitch
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.