I have a few “tricks” I learned many years ago.
To save air, you do two things, First, you breathe in slowly until your lungs full up. Then you release it half as slowly and beyond where you think it is possible. So if it takes me 8 counts to breathe in, then I take 16 counts to let it out. Toward the last few counts, it will feel like you have no air left to let out but you will. Eventually, this style of breathing under water will become so relaxing that it will become automatic and this is how you will want to breathe. It starts to feel like meditation or yogic breathing. I think this skill comes later as you dive more and start to feel more confidant and in control of your body. So I think learning buoyancy as soon as possible is the first step.
To practice buoyancy, you go to the sandy bottom at a depth where you can no longer feel the waves at the surface pulling you around. First, you make sure you are in neutral buoyancy with just enough air in your regulator or preferably no air if you are weighted properly. If you feel yourself drifting up, then you know you have some extra air in your BC and you can release more air or you might need a little more weight.
New scuba divers usually need extra weight because they don’t yet know how to breathe and they tend to drift up, especially when they get anxious. Personally, I think it is okay to have a little extra weight in the beginning over drifting up uncontrollably. A dive master may tell you not to use too much extra weight but I did use extra weight when I started.
Next, you get into a position with your knees on the sand. When you are comfortable on your knees and you are not moving around or struggling too much to stay in position, you can start to practice using your breath to control your buoyancy. First, you breathe out slowly and your breath will bend your upper body toward the sand. Then you breathe back in slowly and your breath will bring you back to your original position. You just play with this way of breathing and it will start to give you a sense of control over your body.
Maybe someone else has some tips to add.
@Sandy VDH anything to add or suggest?