I am currently putting together a quilt for which I have already quilted the blocks. I put the right sides (tops of blocks) together and match points. I sew through all layers except one backing fabric layer, stopping stitching about 1/2" from each end of block.
Trim the batting, but not the backing. Lay the two sewn blocks flat, right sides down. Smooth the sewn seam allowance to one side.
Fold the seam allowance under on the unsewn backing. It will be larger than normal. Pin it so that the fold extends over the previous line of stitching.
Turn over so that tops are up. Stitch in the ditch between the two blocks, reaching under quilt to remove pins. Again, stop 1/2 inch from the end of each piece. This is so that rows of blocks can be sewn together and borders can be added later. (Sew all borders together, then add to quilt.)
Start with a few scraps of fabric sandwiches for practice and you'll see how it goes. Not hard at all. This method was taught me in my first quilt class by none other than Judy Murrah of Jacket Jazz fame!