Place it where it is needed. Whether you need to leave some white spaces, draw irregular edges, or on the contrary, straight ones… Masking is a solution that will make your life easier in a lot of situations!
What accessory can't you live without? Masking tape. It's not very sticky so it doesn't damage the medium when you remove it.
Step 1: attaching the tape.
Step 2: paint your design . Make sure your paint overlaps with the tape, for a perfectly straight edge once the tape is removed.
Step 3: wait a few seconds, then gently remove the tape, without forcing it, before the paint is fully dry.
Note: A thin film will form on the surface of the acrylic paint once it is dry: if you remove the tape too late, you may tear off the film, too.
For example, you can:
Example: to represent a white fence in a field, create a masking tape grid, apply a green wash, then remove the tape.
Don't panic! You can also use a plain piece of paper. It won't be as practical because it isn't sticky, but it works just as well!