Here's the only tricky part in mastering the knitted cast on and knitting in general. You need to slide the right-hand needle back out from behind the left-hand needle, while keeping the loop you just made on the right-hand needle and the slip knot on the left-hand needle.
Keep holding onto the yarn with your right hand so it doesn't get away from you. Slowly slide the right-hand needle down so the loop gets close to the tip of the needle, but does not slide off. At the same time the right-hand needle will be coming out from behind the left-hand needle.
When the right-hand needle just comes out from behind the left-hand needle, give the right needle a little push with your left index finger to push the needle in front of the left needle.