Hey there! Recording Engineering and Mixing are very complicated subjects and take years to master. Just like playing the guitar! If you research some blogs on the subject, they will all say the same thing; you gotta experiment. But I'll try to lend a helping hand as much as I can in this situation.

Layering is a great way to make your tracks sound big. Which technique to use for layering entirely depends on what the instruments sound like. If you are doubling a vocal, you want both vocal tracks to sound identical. If you are doubling a synth, maybe you want one synth to have high frequencies and the other to have low frequencies. Try messing around with different combinations and see what kinds of tones you come up with. Another aspect about layering is panning. The layers will sound different based on where the sound is coming from in your sonic plane. Mess around with panning for different results.

Your second question is a tough one. Balancing multiple instruments so they blend together can be very difficult. The Graphic EQ settings in your instrument's effects are your friend! You'll need to figure out what frequencies each instrument is showcasing, then experiment with combining those frequencies together with other instruments. Sorry if this answer if vague, but them's the breaks!

Mixing is a beast. You'll need to do a lot of experimentation to learn more about how it all works. Of course, check out blog articles for tips and tricks. But my biggest piece of advice is SAVE. Save your file often and with file names which organize your work. Therefore, when you go too far with a mix and can't get back to where you were, you are able to find that old saved file and start again.

I'm sure someone else will chime in with some better advice or more specific instruction. Good luck!