Lay the angled block on top of the block that you're going to slice. Note: both are right sides up. Put your ruler on top of the angled block and slice directly along the edge, cutting thru the bottom block.
This is what it'll look like.
When you join them, you must offset them. A little bit of a triangle should stick up above and one below, by a quarter inch. Sew along the edge of the blocks and when you open them up it will hopefully be fairly straight.