A slip stitch is often used to sew a seam from the outside of a garment or item you are sewing. Most commonly it is used for hems when the seam should remain unseen.
Step 1: Thread your hand sewing 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 to double the length. You have have a lot of extra thread, but that is much better than not having enough to finish your seam.
Step 2: Double the thread over at the needle and cut.
Step 3: Tie a knot to secure the ends.
Step 4: Start the inside of your hem. Hide your knot in the fold of the hem.
Step 5: Right next to where you began, on the other side of the hem, pick up with your needle just a couple threads of the fabric.
Step 6: Next go back down into the fold of the hem. The needle will follow that fold. This will allow the thread to stay hidden as you make the seam.
Step 7: Pull your needle out of the top of the fold.
Step 8: Right next to where your needle comes out of the hem. Pick up a couple threads from the other side.
Step 9: Go back right down into the fold of the hem.
Repeat until you have finished your seam.
When you have finished your seam. Tie a knot to close your fabric, then pull the excess thread back through your stitches without piercing the front of the fabric. After about an inch (2cm), cut the thread off your needle.