Measure your spokes like normal, and add 4mm per full twist. Full twist=spoke going to the hole it should be going to, or 2 half twists.

Easiest way is to lace the wheel normally, and then unlace, twist, and re-lace them in pairs.

As stated above, hopefully this is purely a cosmetic project. I have a few twisted wheels, and they do seem as 'strong' as most of my other wheels, the are constantly needing retensioning. Getting equal spoke tension wth twisted spokes is pretty much a write off.