How to make a precise mask in acrylics. There is a paper with a low-tack adhesive called Anchor Continental Formula III Signblast Tape. You can get it at DickBlick.com. Many people, thanks to Bob Ross, simply call it contact paper. You can create many types of stencils with this paper by using an X-Acto Knife.
For masking with tape, rubber cement and wax, you need to remove them before the acrylics dry. Basically, after about 15 minutes of completing that area of the picture.
Hope this helps you!