After completing the puzzle, first apply waterproof glue to the front. This will hold the puzzle pieces together. Then, for example, you can buy a picture frame, even a floating frame. Apply glue to the back of the puzzle pieces and attach them to the frame. The effect is great; I love the texture of the glued pieces.
If you don't plan to frame it, there's no need to use glue. Simply divide it into several large pieces for easy storage later. Even if you do frame it, if the puzzle pieces fit together well, you can place it directly in the frame and hold it down with a glass plate.
Only consider using glue if the fit is extremely poor (like when lifting a hundred or two pieces will cause it to fall apart). You can apply it to either side, but beginners usually apply it to the front. Make sure to place a glass plate (not cardboard!) underneath to prevent it from sticking to the table or paper.
When applying glue, don't pour too much at once. Use a piece of wood or something similar to spread it evenly, allowing the glue to seep into the gaps between the pieces. Let it sit for a while. The colors of the puzzle pieces will also change slightly after applying glue. Applying glue is a skill, so it's best to practice with a puzzle piece you don't particularly like first. In short, I don't recommend using glue.
