Here’s my idea. I’ve never tried the grid part, but it may just work as long as the design isn’t uber-complex.
1) Get your designs on a piece of paper (or large posterboard-whatever). Draw a grid over the design on the paper.
2) Draw the same size (by size I mean how many squares) grid on the wall, but enlarged. Using a pencil, carefully copy each grid square from the paper to the wall.
3) Check your work. Make sure it looks nice. Erase the grid lines.
4) Use blue painters’ tape (NOT masking tape), cut it to to fit your designs, and tape the part of the design you DO NOT want painted. I would also tape 6in around the outside of the whole design to prevent spillover. It works better to have a lot of tiny pieces of tape overlapping than trying to get one peice to conform to a detailed part of your design. NOTE: Taping will be very painstaking and will probably take a long time. But the better job you do, the nicer it will look. Press down on the tape HARD afterward to make sure it is fully stuck onto the wall. Go over the tape multiple times if needed because paint leakage is a pain in the ass. **Don’t forget a drop cloth on the floor to protect your carpet/hardwood/tile/linoleum from paint drips**
5) Don’t use spray paint to pain the designs. Use canned paint and brush, you’ll be much more precise and it won’t smell up the whole house. Give it multiple THIN coats. Thick coats may make the tape leak, so don’t glob the paint on.
6) Wait 24 hours after the last coat of paint and peel the tape off. Be prepared with white paint to touch up any leaks/mistakes (use tape before fixing mistakes).