In order to do this you will need some newspaper pages or large pieces of paper. Also some jigsaw puzzle glue.
After the jigsaw puzzle is done, push the pieces together tightly. Take the newspaper or any large piece of paper and slide it under the jigsaw puzzle so the puzzle rests on the paper. Again tighten the pieces, especially the edges, together. Take the jigsaw puzzle glue and smear it on the top making sure the edges of the puzzle get covered well. Do not be worried if the jigsaw puzzle glue is white in color. When it dries it will be clear. After leaving the puzzle to dry overnight liberally smear on another coat. Tear off the excess paper around the puzzle and slide any hard board under the jigsaw puzzle. You are now ready to lift it for framing yourself or take it to a framer to be finished.
To find jigsaw puzzles and the glue go to http://www.hobbymasters.com and order what you want.