When it comes to the mixing and mastering process, I really don't do too much but I do have several simple rules to follow.
1. Keep your levels low at first.
We want to mix music with low levels because during the mastering process those levels will be brought up a lot. Most of my levels are between -16db and -8db.
2. Make sure your Master track does not peak out. (hit red)
If we keep our levels low we won't have this problem, but you want to make sure that your master stays just under peaking levels so that when the mastering process begins you don't have any distortion. You'll have to spend time playing through the entire song and watching for the peaks to happen.
3. Use a brick wall compressor.
Using a brick wall compressor will bring your levels up to the radio volume that everyone wants. There is a brick wall preset in almost every compressor and there is a stock compressor that comes with almost every DAW.
4. Use a high pass and low pass filter.
The high pass and low pass will get rid of any unwanted extreme high and low frequencies.
I would add a #5 and say use mastering software, but I know you can accomplish the same things I've done with an EQ and various other compressors. It doesn't hurt to have mastering software if you're not quite sure what you're doing.