Once you remove the surface from the existing frame (assuming you will do that), if you lay it down flat (face down) on a smooth hard surface...like a garage floor...you should be able to pound a few of the dents out decently.

If you attach it to a wood frame, you should also be able to adjust it some to get the surface level. Remember to leave a little room around the entire edge of the field (about 1/16 to 1/8") when you are making your frame. Then when you put the screws in to hold the field, it will stretch that little bit...which should also help level it some and make it fairly stiff.

You may have seen this before...but I explain in some detail what I did here:
http://www.miniaturefootball.com/for...ecommendations