Angiel Hey man, thanks for the comment and also for finding the time to play NMP. I am really glad that you found the tutorial level to be useful. That is some sweet feedback for my ears.
Now onto your questions:
I do all the work for NMP except the music, which is done by a friend of mine. When I started, I had almost no experience in pixel art animation, but I do make games for more than 10 years now, so transition was fairly quick for me. I also come from animation background and that helps.
If I have to give you an advice I will say, go look at others people works. Find artists that you like and study their work. Here are some that I like Paul Robertson or Miquelito
I can also recommend that you go and take a look at This thread. There some really insightful stuff people post.
The other thing is hard work and those 10000 hours.
Not sure this was of any use to you, but if you have any pixel art related questions just pm me, and I will try to help out.