Its a matter of practice. Just know that as long as you only move the razor in a straight line and dont wobble it side to side, you cant fuck up too bad. You want to learn to hold the razor such that it is always meeting the surface of your skin at a 45 degree angle, use very short strokes, and use your other hand to pull your skin taught.

You also want to remember that, especially when starting out, you dont have to do a great job on the first or even second pass. Do the best you can, and if there is still hair, just re-apply shaving cream and go over those spots again. Eventually you get good enough you can get everything in a single pass most of the time.

There are some good videos on youtube where people have tutorials on learning to use one. Also because it cuts closer to the skin, you may want to invest in a face moisturizer in addition to an aftershave. Really helps to prevent ingrown hairs and makes your face feel amazing after shaving with one. For pubes/longer hair, I would shave with an electric shaver first to shorten the hairs, and then follow up with the razor.