How to Convert a Regular Plaid Pattern to a Tartan Plaid (and Keep it Seamless!)

A regular plaid is an intersecting pattern of stripes running horizontally and vertically. A tartan plaid is an intersecting pattern of stripes running diagonally. If you've ever had a regular plaid that you wanted to turn into a tartan plaid, you may have had trouble trying to get the rotated pattern to stay seamless. This tutorial will show you how it's done.

I'm using Photoshop for this tutorial, but most image editors will have the necessary functions to do this. This will only work with square patterns.