I trim the seam allowance on the fabric before adding the next piece. That way, you can line up the edge of the added fabric to the edge of the fabric already on the block. For the block, trim it just before you sew it to the next block. I remove the paper from the seam allowance once it is sewn. I leave the paper on until that block is surrounded. That means the the outside blocks would have their paper until the borders are added.