I do a lot of tailoring, and here's now I match stripes. Take a piece of paper cut to the size of the finished welt. No seam allowances, just the part that will show when it's finished. Place it on the fabric where it will go. Use a pencil and mark on the paper where the pattern enters and leaves the paper. In other words, trace the tartan pattern on the paper. Then put the paper on your fabric for the welt, and move it around until the pattern on the paper matches the pattern on the fabric. Trace around the paper with chalk, add seam allowances, and now you're all set! Obviously for a single welt you'll need to add the other half of the folded welt, but mark the half which will be showing with chalk or a tailor tack so you remember which side to use.

Does that make sense?