Be sure to watch the video made by our friend Peter: https://youtu.be/UlJYP1hJCmw
Other than that, just practice. You could try angle guides (we do sell one), but they're just a crutch and like training wheels, not really a great idea in the long run. Just practice and watch and look and feel as you go, and most people will improve pretty quickly to where it's not an issue and you get the knife plenty sharp. Best not to obsess about the exact angle... like he says in the video, you can use the usual trick of painting the edge of the knife with a sharpie, then as you start sharpening, you can see if you're getting the angle right by how the sharpie comes off (evenly, or too much towards the tip or away from it...).