I'm trying using Burn/Expose + Smuge tools. I think I'm having problems with the teeth, as I can't seem to find a way to give them a good size. I'm currently working with skull references, but I can't get to final decision on how to work them.
Anyways, if I seem to be missing something, I did appreciate any advice.
Overall the proportions and form of your skull does not look correct (which is a bigger problem than the teeth, though also related to the teeth size) and if you're trying to do that sort of "skull being visible under skin" type of x-ray pic, you're ignoring all the room the muscles and fat in the face.
The dodge/burn and smudge/blur tools are a surefire way to make a digital painting look amateurish. Oh, the pros can use them (sparingly, as and when necessary) and make it look masterful, but in the hands of a beginner, they're tragic.
I was once on the receiving end of a critique so savagely nasty, I marched straight out of class to the office and changed my major (sketchbook).
Oh, thanks for the advice. I'll try to get more skull references. On the other sides, what other PSD tools could I use?
The best way is that you try to learn to paint only using the round/square hard brush/soft brush and picking the colours by sight, do not depend on other tools that make effects since you don't actually know/understand what they do when you are starting, so you will end up getting effects that aren't how they are supposed to be.
Just rely on the default brushes, change the size/flow and the opacity, any masterpiece can be done with that. If you don't get the result right (the one you are expecting), it's not the tools fault, it's actually your lack of knowledge (that's what I always tell to myself). So just do as @TinyBird says, try to paint as accurate as you can photos or your own teeth, try 50-100 times, until you get a grasp, and then go back and try (that's how's supposed to work).
Just rely on the default hard brush (round/square, and soft for blending), control the size and the opacity. Pick colours from sight, but for doing so you must first learn how light works. As @Tinybird says, do studies (until you get a grasp) and then try again. If you don't get the result you are expecting it's not because of the tool you use, it's because of your lack of knowledge (that's what I always tell to myself).
Try to avoid other tools at first and effects since you actually won't understand what they do or how you are supposed to use them (although it's always a good idea to experiment with them, to try to see how they work, don't rely on them to create something unless you have a good understanding of a lot of things).