There are a couple ways I'd go about this, depending on the pattern. I'm hoping it's just small squares.
If they're random (like a postage-stamp quilt), then I could not recommend better than Billie Lauder's Quick Quilts where she shows the mix of scrap pieces, dividing, resewing, recutting, resewing, etc. until you end up with 1-1/2" pieces from 5" squares. I recently did a 3,400 piece tablecloth like this and it's lovely.
If, however, there is a definite pattern to make with those pieces, get some fusible that's already lined in 1-1/2" squares (or draw your own - pencil on the non-glue side). Arrange your unsewn squares where they need to be placed on the fusible side, tacking with a mini-iron just enough to hold the piece in place. When all is where you want it, press flat with a regular iron to firmly fuse all raw edge pieces to it. Then, seam from the back like you would a watercolor quilt and you got it. = No matching corners, no individual seams, etc.
Hope this helps. I'm sure you're going to do a fabulous job.