This is probably the easiest (but not necessarily the best) cast on method.
1. Start with a slip knot
on one needle.
2. Wrap the working yarn (the yarn attached to the ball) around your left thumb (see picture below). I like to put my right index finger on top of the stitches already cast on, just to hold them in place.
3. Insert Needle into yarn on thumb
4. Drop your thumb and pull the loop onto the needle
NOTE: If you're having a difficult time wrapping it around your thumb and all that, you could just fold the yarn into a loop (making sure it looks like the loop in Step 4 - with the working yarn coming to the front) and place it onto the needle.
5. Pull tight
6. Repeat for as many stitches as needed.
As I mentioned before, this isn't the best cast on method because it creates rather tight loops that make the first row rather tricky to knit. Also, it stretches out as you knit the first row. This cast on is good to use in the middle of a project when the pattern calls for adding on a couple stitches.
Please feel free to let me know if any of this doesn't make sense or you have any questions. I can always take more pictures or add more descriptions to make this more clear.