| Combination of stencils purpose made to block in the major sections of the picture, then touchups by hand or whatnot.
The real trick is any time you do this, you have to go in with the knowledge that for the most part your grille cloth is vinyl based. Paint doesn't readily stick to it, you cannot treat it with anything that will solve that because you'll melt the weave... so you have to use either special paints that etch and cling, or you have to use a different medium for cloth.
If you use special paint, it gets a bit costly, and you still have to handle with care.
If you use special or plain different cloth, you have to contend with the fact the material you're using won't be acoustically transparent, will load paint that will make it moreso, and it will affect your tone. Nothing that can't be compensated for, but....
The majority of things I've done haven't been too intensive as far as little details, because they get lost on the audience. I usually just bring it to a point where the design is coherent and attractive...
The way your cab is made with a metal grille is prolly much different than the way a traditional wood / cloth grille installs in a similar cab. Without seeing the design of yours, it's hard to say. Post some pics. If it's possible and you're interested, I'd be happy to do the work you have in mind.
For a little background, I have a side business where I make 100% custom cabs. I do custom grilles too, including artwork. I have no "models" in my line. Each is a one of a kind based on the customer wants. I also restore cabs. You want a 2 x 12 that looks like a 30's radio? No problem. Gimmie a holler if and when you're ready. | 
