Position the Wavemaster punching bag where you want it. It's easy to move now, but later, when it's filled with water or sand, it will be much more difficult.
Find and unscrew the cap on the bag's plastic core. The cap is usually near the top of the base. On some models it may be on the very top of the core. If this is the case, you may need to remove the padding from the bag to get to the cap.
Find a hose long enough to reach from the nearest water source to the bag. Again, if you have to move the bag from its resting place before it's filled, it will be pretty hard to move back when it's full.
Fill the bag with as much water as possible. Using as much water as you can ensures that the bag will weigh enough to remain stable during your workout.
Screw the cap back on and, if necessary, replace the foam padding.