Installing a grille cloth is not an easy thing to do and get right.
Be care with the heat. Warming the grille cloth a little will help it bend to make a nice sharp edge. Heat it too much and the material shrinks. Heating it enough to shrink the cloth can be done after all the staples are in place to shrink the grille cloth so it is tight against the frame. If it is too loose, it can flap when you are playing.
One way to fix the sag would be to remove the staples from the end, lightly clamp a board against the top end on the front of the frame at each end, pull the grille cloth tight, tighten the clamps. This takes some co-ordination to get it locked down with the grille cloth tight. Inspect the front, making sure that it looks right and is pulled evenly. Then bend and pull the fabric over the edge and staple it. Sometimes I use another board pressed up against the edge of the frame to get a tight bend.
If you don't have enough material to pull the cloth because it is trimmed, you can buy fabric stretchers or metal bending pliers (also called seamers) like in the pic below.
The other thing that you can do it take it to an upholsterer and have them fix it for you. They have the proper tools and the skill to do it quickly. It should't cost that much at this point. Less than buying clamps and pliers.