Well. Never tried this with .357... Mostly with .41 &.45 LONG
Colt.
Anyway, cut some wads from a cleaned & dry milk carton. You'll need one over the powder, and one over the shot.
You probably won't be able to get much shot into a .357 round, but look up the lightest load for the lightest bullet in a fast powder. Bullseye. Red Dot, etc.
Load that into a primed case.
Slide a wad down onto it. A dowel helps.
Unless you have some loose shot, sacrifice a few shotshells to get it. I find a mix of 6 & 8 or9 birdshot works. Fill the case to just short of the mouth, add a second wad, and seal with a dab of silicone sealer.
Remember, you're not looking for 1400 fps here, 450-500 will work fine. Test loads on a cardboard box to see if you have "buzzworm coverage".
Don't shoot yourself in the foot...
Tom