Here is a grid I made using this technique if you want to be lazy and just use mine:
Grid!
I spent a long time scouring the web trying to find a glorious tutorial on how to make gridded crafting patterns (specifically for cross stitch) using Photoshop with little to no avail (I did find several very expensive programs to do it, however). Of course you'll have to use a color matcher or guesstimate when translating screen colors into beads, threads, yarn, etc.
