A small walkthrough of my speedpainting of minotaur which I did a long time ago. Just thought to show how I work with speedpaints- it's also similar how I do in traditional medium.
When you do speedpaints- if something didn't turn right, don't give up. Finish it. You don't have to put all your energy to make it look the best. It's a practice after all. Practice means to draw, learn, study and it's better to make 50 pieces of so-so looking practices than 5 pieces of amazing looking and given with your best (and lots of rubbing with eraser!) ones. Because in that way it will easily discourage you.
While practicing, always use reference- they are for learning after all. If you do lots of, even ugly ones, your mind memorizes shapes. After 30 fast drawings you'll notice your change clearly. Just too many artists put their best in one speedpaint piece and focus too much of how it didn't turn it out right.