I think I'm with Jan - I would probably just deliberately put it on at a 30-ish degree angle or so. THERE, it's SUPPOSED to be slanty! LOL
If I NEEDED straight stripes...I'd probably starch the bejeebus out of the backing and tape it firmly down to a table, aligned so that I could use the table edges as a guide to feel through the layers to tell what is and isn't straight, and then use a ton of glue to hold it all together. I've never done that though, so it's only a theory.
