Step 1. Buy some expanding foam. http://www.uscomposites.com/foam.html
4lb density works well for this application.
Step 2. Cut or drill a hole in the bottom of a plastic decoy...preferably towards the tail. If there is any water in the decoy, drain it and let it dry out for a few days.
Step 3. Patch any shot holes with electrical tape.
Step 4. Mix 2-part foam per the directions included by the mfg. Some are wt/wt and some are vol/vol. If you get it close, you'll be OK. Make sure to stir very well. Depending on temperature, the foam starts to expand in a minute or so.
Step 5. Pour foam into hole you cut in the bottom of the decoy. Hold the decoy so that the hole is pretty close to the highest point so that air can escape (not critical, but the best way to do it).
Step 6. When foam gets close to coming out of the hole, hold a piece of plastic over the hole (trashbag works well). When expansion is complete, you can take your hand off the plastic. In 10 minutes or so, the foam will have cured and you can peel the plastic off. If you peel it too soon, you'll have foam stuck to it. Trim any excess foam with a knife or sandpaper after it has fully cured. Peel the tape off any repaired shot holes.
Step 7. (Optional) Patch the foam hole with bondo or similar material. If the decoy is fully filled, there is no need for this. However, my father-in-law thinks it is a good idea. Since he's retired and plenty of time on his hands, I let him patch the holes.
Remember the foam expands to approximately 15x its liquid volume. It expands less and slower the colder the temp and more and faster the warmer it is. If you do as I do and drill the hole at the tail of the bird and hold it with the head pointed down while filling, you can add more foam through that hole if you fail to mix enough to fully fill the decoy on the first shot.