Big help bro! Made so much sense to me. I've only been the artist my whole music life and this past year I have began recording my self and others. I didn't go to school for it. Just YouTube and natural music ability..but You can't use God given talent to escape theory lol1. When you are mixing your beat, you need to leave room for your vocals. Basic physics says that two things cant occupy the same space. So, no two similar sounds can be in the same place on the mix which is where panning and levels come in. So your hats and mid to high freq sounds can't be in the same place. Pan them around. You will often be able to make something louder in the mix by panning rather then raising the level.
2. When you are creating your mix, you need to leave a place in your mix for the vocals. Vocals (depending on the vocalist and whether we are talking about singing or rapping) land in the mid to high freq range, so you need a place in the mix that doesn't have other sounds with similar freq's panned and leveled to the same place. This is where your vocals will go, even if you don't have them yet.
3. If you are giving you beat to someone to listen to and possibly use, that demo needs to be mastered. Otherwise it will sound quite and flat, and they won't want to use it. However, when you add the vocals, you use the pre-mastered version of the beat, and master the beat and the vocals together when you are done mixing. Mastering on top of an already mastered beat will not sound clean and cohesive which is your goal.