As 5bugles has said, you have to experiment to find the correct position for the tapes or "sweet spot." Once you find your sweet spot you should be able to get it right every time. I cannot remember the last time I had to retype a diaper because I taped it wrong.
Many diapers have visible marks that you can use as "targets" in order to then position pin the tapes the same way each time. Look for lines, name labels, the edges of the absorbent mat, the marks you can see in the plastic from the standing leak guards, etc.
Practice in front of a mirror to learn how to tape to your "targets." Then pay attention to the angle of the tapes and how much tension you are applying. You then should be able to tape the diaper "by feel" in a public restroom without the benefit of a mirror and get it right the first time.
--John