On the real dive, before you enter the water, have some air in your BC vest - nowhere near full - so that you affirmatively float. Jump in & check all your gear. Then remove all the air from your BC, take a full breath and check your buoyancy. You should float with the surface near eye level. Exhale fully & you should sink a foot or so.
To descend, since you have almost no BC air left to dispel, exhale fully, aim your body downwards and swim down with your fins. To reduce ear equalization problems, don't go straight down; go at an angle. If your target is straight down, or you have to follow an anchor line, then make zig-zags or circles. As you descend, your lungs, neoprene and the minimal air in your BC compress so that you become more negatively buoyant and you naturally stay down. With little or no neoprene, you may need no BC adjustment. Or, you may need to add a little air to keep from hitting the bottom.
When at the desired depth, to hold that depth, try to do so by aiming your body up or down and finning there, while making small adjustments with your breathing. To ascend, aim your body up and fin there. Remember not to shoot upwards in a hurry, because your neoprene & lungs will expand, make you lighter, and you will have little air in your BC to vent. Thus, you will run the risk of an uncontrolled ascent and the bends. To avoid that danger, be ready to aim partially back downward and fin that way.
In general, try to avoid adding or removing air from your BC, because that siphons off tank air from more important uses ' like breathing! BC air also makes you less buoyantly stable so that you keep bobbing up & down. And, the bother of frequent adjustments detracts from enjoying the ocean environment. Often neophyte divers turn BC air into a buoyancy crutch. The BC is more of a safety device for the surface, and a place to hold our gear. To help break the habit of pressing the air button, we should remember that the first generation of scuba divers did not even have BC's, but they could go where they wanted underwater.