First, you're going to start by making a slip knot. Keep the cut end long - if you're going to be casting on a lot of stitches, the longer the better. You can always trim the excess. Again, you'll want to make sure there's plenty of slack after the slip knot so that you can cast on more stitches. The loop should be connected to the yarn ball.
Cast on the first row by looping the yarn end around your left thumb. Slide it onto the knitting needle. Loop the yarn from the yarn ball around the needle with your right hand, and loop the left hand's loop over the right hand's loop to complete the stitch. Continue casting on as many stitches as you need for your project.
Click below to activate a video tutorial demonstrating a two strand cast on: