For me it's:
1. Measure
2. Measure Again
3. Cut
4. Align
5. Stretch and Staple
6. Trim excess if needed
That's pretty much it. Just look at how the original cloth is attached and try to copy that. The edges and corners don't matter much since you can't see them when it's installed. It just needs to be straight and pulled tight across the front so it doesn't flap arround. My MKIV has small pieces of felt between the wicker and the grill frame to give it room to vibrate without slapping against the frame and making an irritating buzzing noise.
Mojotone has a pretty good supply and good prices. No exact Boogie replacements, but nice Fender and Marshall style cloths.
http://www.mojomusicalsupply.com/