Sketch out the image on paper. Work out the design elements and color combination until they look like the image you imagined.
Transfer the sketch to your canvas with pencil. Use a ruler to draw straight lines. Focus on getting the proportions right.
Block in areas of color. For straight, hard edges use masking tape and let one color dry before adding the adjacent color. Let the paint dry and repaint to fix problem areas.
Mix and blend colors to create softer edges and a greater sense of depth. The work of Mark Rothko is a good example of preplanned abstract painting with soft edges and blended colors.
Stop when the painting is close to your original idea, but don't expect the it to turn out exactly as you had imagined. Sometimes what seems like a mistake actually adds vitality and interest to a painting.
Set up your workspace for easy access to paint, brushes and tools. Put out plenty of paint and keep everything in easy reach so you can be spontaneous.
Grab any tool you are drawn to, load it with paint and mark the canvas without too much thought. Work with various types of brushes, pallet knifes, rags, and even your hands. Move according to your impulses and emotions. Swipe the brush across the canvas or move slowly and deliberately.
Paint quickly to blend colors on the canvas. Remember acrylic paint can dry within minutes. Spray the painting with water to keep it workable longer or to create dripping effects. Acrylic gel retarder will also slow down the drying process.
Squeeze out more paint as soon as you need it. Choose whatever colors you are drawn to. Follow your instincts. If you have an impulse, just do it without question. Work as quickly as the ideas and impulses come. Stop when you can't see what to do next. Walk away and do something else, then come back fresh and look again.
Stop painting when you are happy with the results and you have no desire to change anything. Only you know when your painting is finished.