Acrylics are a great medium to work with as they are rather forgiving, you can easily correct mistakes by simply painting over them. I always start by drawing a fairly detailed sketch with pencils...usually one can still see the lines after grounding and that helps a lot. Then I start by grounding the different areas and already try to shade things roughly. I always colour the whole canvas before starting on any details to see if the colours will look good together. As one can (almost) always paint over layers of colour already on the canvas it is best to paint from the background to the foreground so that you don´t have to paint around objects. That means that one finishes the background before one starts to add details to the creatures/objects in the foreground. I tend to work with small quantities of paint and I mix all tones from the 5 basic colours...that way the shades and tones vary a little every time I have to mix the colour again creating a wider variety of tones. It is important to know that (most) acrylics tend to dry a little darker so the tones you use have to be a tad lighter than you want them to be in the finished painting. The amount of water used is very important, depending on what effect you want to create and what brush you use...one has to experiment a little to get that right. It is also important to know what kind of brush to use for what as they hold different amounts of colour, this takes a bit of experimenting again.
That´s all I can think of right now...I hope this helps you. Feel free to ask if you have any questions.