While a lot can be learned in machining from studying books, I have always found that one of the best resources is older machinists. They can explain a lot quicker how to get something done than you can read about a technique and try to figure out what the author was trying to say. They will also probably sprinkle in some insults about your dubious intelligence, which are good to save for later use. With that in mind, here is a good video primer on using a surface grinder to square a block, done by Don Bailey, owner of Suburban Tool in Auburn Hills, MI (near my hometown!).
Don sure does seem to know a lot about Zig Zags…. he doesn’t really LOOK like your typical Zig Zag user, though. But his advice to roll your doobs tight… no wait, I mean, to use cigarette paper as a good .001 shim on your surface grinder is good advice. Paper is very slightly squishy (there isn’t much to compress, obviously, but it will compress just slightly, especially when wet), so it can give you that “just enough” amount of shim on a surface grinder project.
Don also points out to go no more than .0005 on a surface grinder. Go too heavy, and you’ll load up the wheel too much, burn your material, break the wheel or at the least make it break down quickly, giving a bad, uneven surface finish, which is pretty much the opposite of what you want when using a surface grinder! Now, that advice is for your typical small surface grinder– larger ones no doubt can take much heavier cuts, as they are designed for that.