There are tons of pictures - I promise it's the best way to learn!
To practice slip stitch you'll want to cut out a small square of fabric. Fold it in half and press. Open, and then fold each side into the center line. Press. Fold down center line. You'll have something that looks just like the piece of fabric I'm using.
Now, thread your needle - I chose double threaded for this one because it makes it more visible! Double threading your needle will make a stronger seam, but choose single threaded if you want the stitches to be almost invisible. I'm also using purple thread, though typically you will use thread to match your fabric so it blends in. :)
Knot it and pull the needle through the top of the fabric by sticking it through the top folded edge. Pull the thread until the knot catches. Now, move the needle to the bottom fold. You will want to stick the needle into the crease horizontally and push it along behind the fabric for about 1/4 inch. Then, pull the needle through and go back up to the top fold. Push the needle horizontally through the top fold and pull through. Continue this until the end.
At the end, make a small stitch in the fold opposite of your last long stitch. Pull the thread until it forms a small loop. Put your needle through the loop twice. Pull on the thread to form a knot. Do this twice for extra staying power. :)
The pictures at the end show what the slip stitch should look like. Pretty tiny, right? Now imagine that with matching thread! Lovely!