What's proper way to grease trailer wheel bearing?

If you found water in the hub, you should REMOVE the inner and outer bearing from the hub, clean them in solvent, inspect them for rust and wear, repack or replace them and put it all back together, after you clean out the hub…If you need to replace a bearing, be sure to replace its race…

Since you have no idea as to the condition of the bearings, just pumping grease through them does nothing…You need to clean and inspect them to be sure they are serviceable…