If the blocks are complete join with a fancy wide zigzag, think joining two seams. If not finished, 2ways.
1. Top of block just machine join, ensuring areas match. Turn to back flatten front seam and hand join batting. Now turn over the backing easy to hand stitch .
2. With sashing. 2inch wide strips sew both onto one block you can sew on batting but I prefer next step. Top downwards and hand sew strip of batting to block . Take 2nd block and sew onto top sashing. Hand sew batting I place. Now just hand stitch into place. Then quilt stashing if you wish.