The first step is working out how much yarn to leave for the tail. If you know how many stitches you need to cast on you can wrap the yarn around your needle once for every stitch needed, which should show quite accurately how much yarn to leave for the tail. You can then just hold onto the yarn at the point you've calculated for where the tail should end and slide the loops off your needle. If you're not sure exactly how many stitches you need, you can leave a bit more yarn for the tail to ensure you have enough.
If you don't know how many stitches you need to cast on I would recommend casting on ten or so stitches as a practice. Based on how that looks you can probably work out how many stitches you need and can then unravel the practice cast on and start again.
Next you need to make a slip knot, hold onto the yarn with both of your hands at either side of the point calculated in the first step, the tail (and end of your yarn) should be on the left, with your ball of yarn being on the right.
Place the yarn you're holding in your right hand on top of the yarn you're holding in your left hand, which creates a loop. Hold onto this with your left hand.
Take your other hand around the back of the loop and put your thumb and index finger through the loop and grab yarn from the tail.
Pull the yarn slightly back through the loop and pull on the strands of yarn as you do this, so you've created a new loop and closed the original loop. This loop is your slip knot.
Take your needle in your right hand and place your loop onto your needle, with the tail of yarn at the front. Pull on the tail to tighten up this loop on the needle. This counts as the first stitch.
You should have two strands of yarn coming from your needle, with the tail being at the front. Hold onto your needle with your right hand and put your left thumb and index finger between the two strands of yarn and grip the two strands together with your other fingers.
Part your thumb and index finger and bring them upwards, so you can see there's two loops, one going around your index finger and one going around your thumb, with the tip of your needle in the middle.
Bring the tip of the needle to the left of the loop on your thumb.
Take the needle under the left strand of the loop and through the centre of the loop.
Take the needle down through the middle of the loop on your index finger and to the left, so it goes under the strand of yarn.
Take the loop on your thumb and place it onto the end of the needle, so that the loop is now sitting on the needle.
Remove your thumb and index finger from the loops and pull on the two strands of yarn to tighten up the stitch.
Repeat steps seven to thirteen to cast on all the stitches you need.