I use thumb tacks to align backing. but you need to be very careful. From the right side of the backing, insert some thumb tacks along one or two stripes, or whatever you are trying to align. I usually put a small piece of blue painters tape over the head of the tacks to keep them in place.
Flip the backing over and tape the edges down. Put the batting on top, pushing the tacks thru, then VERY CAREFULLY put the top on using the tacks to align the bottom & top. Pin then remove the tacks right away so the blue tape doesn't adhere to the backing.
And I wouldn't trim anything until you are done.