If you research on the web, there seem to be many ways to make a slipknot. Keep that in mind if you don't like my way. I feel it doesn't look as neat as some others, but this is a method which I have taught to younger children who find it the easiest. Everybody learns differently, so if this doesn't work for you feel free to investigate other ways. I promise I won't be offended.
Start with a simple loop in your yarn.
Stick you thumb and forefinger through this loop.
With your thumb and finger, grab the piece of yarn that is the tail.
While holding on to the end of the tail and your working yarn, pull on this new loop until the first loop tightens around it.