It is hard to do if you don't see it well. Ill explain knitting front and back and you can just reverse the process.
Begin to knit a stitch, BUT before you remove the old loop off of the left needle
Move your right needle behind the left needle and go through the back of the old stitch on the left needle. Now knit into that stitch too. Then slide it off like normal. This results in an extra stitch with a small "bump" on one of them.
So to pfb, you just reverse this process.
Watching the video will help.