Your phone is a great piece of equipment and designed as an all-rounder, capable of helping you with many things in your life. Unfortunately, taking good close-ups or "macro" images isn't one of them. The lens hardware in your camera just isn't designed to do this. You'll find that, as you get closer to your skin the focus will begin to blur and the auto-focus will often cycle as it tries to fix the problem. Here's a few tips to help you take crisper, clearer images:
Always make sure you have a good source of diffuse natural light. Diffuse lighting meaning that the room is well-lit but you are not in direct light which can worsen the situation by casting shadows.
Lock your auto-focus to prevent cycling. When you're ready to take your photo, press-and-hold on the focal point for 2 seconds. You should then see that as you move the camera the auto-focus will not respond and you can adjust for a better picture.
Unfortunately, neither of the above changes will solve the real problem which is that the lens in your phone isn't designed for the kind of close-up images you need when monitoring your skin.
A clip-on macro lens is the best solution to this problem. We've done our research and found the Olloclip macro lens to be the best solution, both in functionality and value. This really is the silver bullet for distance, focus and speed issues when taking photos of your skin.