Don't know how much help this is but anyway. Here's some of my thoughts (started painting in the early 90s, had a long pause between 2002 and last year when I started again).
- Don't be afraid to experiment. If it turns out really botched you can always strip the paint and start again.
- If you try out a paint scheme for a unit, paint only one miniature first to completion to find out any problems. If it looks ok, paint the rest of the unit in batches of 5-10 to get a uniformed look. In a unit of 20-40 that one one miniature wont stand out too much. Certain armies are better here than others. Undead, chaos, orks, goblins... there's no problem with them looking a bit rag-tag, shabby, or odd looking.
- If a unit doesn't look completely perfect, just try to find the things that did work and move on to the next. The only way to get better is painting, painting, and painting some more. Mistakes, trials and errors are part of that process. Your going for the long haul here and if you continue with the hobby, then you are quite likely to start several armies. The first ones aren't going to be the best looking ones, so see them as training for say the third or fourth army and make those later armies the ones where you try to do your very best.