Keep in mind that I work from biggest brush and loosest strokes to the smallest brush and finest details. This should explain the secrets of quickly completing a commission in acrylic more clearly;)
1) sketch out and block in your design. Fill in with a watered down wash of the final color, very rough and quick. This step defines the shape you're going to paint.
The next steps add mass and form: You will work from dark to light. (the reason is darker colors are more transparent and white is the only true opaque color in paint)
2) Block in your dark tones & the shadow areas but leave areas from layer 1 showing thru near the edges (it enhances the finished painting)
3) Apply your main colors (the middle tone) use a medium size brush with gestural brush strokes to give it life, dont just block it in or it will read as flat (unless that is what you want)
4) Add in all your lighter tones and highlights.
any additional textures or patterns that may be present.
Give your eyes a rest, sleep on it and finish the painting off tomorrow.......
5) Revisit your darkest tone and lightest tone and use the smallest brush you like to give it crisp details and edges where you want the viewer to look the most (so not the background)
6) Don't forget to sign it!
Hopefully this process will help you create your next work of art!