Lay the new shoes into the drum like they would be if installed. Hold them up to the drum friction surface. Confirm that shoe slightly rocks in the drum, that is that when the center of the shoe is touching that the ends don't quite touch.
If the ends touch and the center doesn't, you will have problems. The shoes need to be arced to better match the drum.
There is a trick to doing this if you don't have access to a brake shoe arcing machine. You can slightly bend the shoe. Place the bottom of the shoe on a solid surface (concrete floor), take a hammer and hit the top of the shoe a time or two to reduce the arc. Don't hit too hard, it doesn't take much. Do a trial fit in the drum again and repeat if needed.
It may also help to champher the ends if it doesn't already have it done. Use a file or a sanding disc.
Make sure that you lube the contact points on the backing plates.
