The first thing to do is check if it is even legal to use.

If it can trap an air breather. Like turtles or water rats etc. Then chances are it is illegal to use. It looks like it is fully closed and no where for an air breather to get out.

In most cases just the weight of the trap (plus bait) is enough to sink a trap and hold it in place. But you can also just add some stones or lead sinkers.

You can tie some stones or sinkers around the outer edge to make it sink evenly. But it is very common just to put some loose stones in.