Painting a flag without paint bleeding Background story: I bought a 3'x5' cotton flag and I painted it as seen in the photo. I made a mockup in photoshop and used a big T-square and pencil to sketch the shape of the numbers. They turned out super parallel, very straight, and very accurately sized.

I covered the edges with painters tape and filled in the tricky white areas with painters tape also. I taped down a trash bag on the larger white areas to save time and lots of tape. I then sprayed the flag with spray paint purposed for fabrics. After I peeled off the tape I was disappointed that there were spots I thought had been sealed but paint snuck underneath the tape leaving blue spots on the flag (which I couldn't get rid off). In other places, the tape apparently didn't stick too well to the fabric so paint bled past and ruined my straight lines. 

In conclusion: I want to make another flag, but without making these same mistakes. What should I use as a better alternative to painters tape on the edges? Wax paper? I'm open to any ideas that may help me avoid this problem next time I paint.
Your best option for "precision" may be a craft known as batik. It involves applying melted wax onto the fabric in areas you wish to remain clear of the color currently being created. This effectively creates a stencil for the portion to be colored. Either paint or dye would then seep into the fabric.

The wax is then removed/washed out and new wax applied over the recently colored areas as well as any others to reject the latest color. As with many things organic and not particularly technological, you will get some less-than-perfect lines, but very likely much cleaner than with paint and tape.

I'd known of the process since my early years (so last century) but other than admiring others' work, never explored it. Your query and my resultant searches have me astonished at the development of the craft. Rather than drop a handful of links here in the forum, I suggest that you use your favorite or perhaps your second favorite search engine for "batik methods" and/or "batik supplies."

I'm going to pursue some of the links for my own entertainment. Another tool for the toolbox for me!