I'd recommend the videos of The Recording Revolution on YouTube. The concepts are well explained and clear. He uses ProTools but the principles can be easily ported to any DAW, including Audiotool. Here's a video about the core principles of mixing: (link is only visible to registered users) and here's the first part of a series about mixing a song from scratch: (link is only visible to registered users) I find that the best way to learn mixing an mastering is to actually watch a pro doing it, and hearing the results of each step. Here's a video explaining the differences between mixing and mastering: (link is only visible to registered users) and a quick 10 minutes mastering session: (link is only visible to registered users) I hope this helps.
Thanks, very helpful.