I just pin and sew using at least a half inch seam. I don't bother too much about matching the pattern - just try and make sure its all going in the same way if it is a directional print. Always works out fine. I've only used a plaid in a backing once, and even though it didn't quite match up on the seam line, it still looked ok once it was quilted. I doubt if you could get a plaid to match perfectly anyway, because most aren't quite straight on grain, so don't stress!