If you have to ask, maybe you should have it done by the LDS. It is not hard to do, but it is possible to strip the threads, apply excessive torque and damage the fitting, or leave it too loose and have it come loose during the dive. If you are not used to tightening things with wrenches, leave it to the professionals. But if you want to do it yourself (I have been doing this sort of thing myself for the past 40 years without problems) hold the wrench with your hand close to the fitting so that you don't get a lot of leverage, and then turn it firmly but not with all your strength. That should get it within a reasonable torque range.