Diver0001 and neilwood already said, but I don't think the importance can be overstated, so I'll say it again.
Get your trim dialed in. Trim is not so much a skill as it is simply a configuration issue.
If your trim is perfect, then you can hover in the water and stay perfectly horizontal while being perfectly motionless.
If your buoyancy control is less than perfect, you may float up or down. But, if your trim is perfect, you will be able to stay horizontal and motionless as you do it.
Try holding perfectly still, while maintaining proper form. You will probably start to slowly tip in one direction or another. Maybe turning head down. Maybe turning head up. Whichever way it is, you can move weight around to achieve balance. Once you get the weight balanced, then you can cause yourself to rotate in either direction simply by extending or pulling in your arms and/or legs to change your center of gravity.
If you CAN'T stay horizontal while being perfectly still, then that means you have to constantly fin or swim with your hands just to keep yourself horizontal. And that means you're burning extra air.
Get your trim weight and practice being completely motionless while being perfectly horizontal. You'll use less gas when you're sitting still. And you'll use less gas when you are finning to move yourself because your body will be more streamlined to the water and your finning can all go towards moving in the direction you want, instead of some percentage of each fin thrust going towards maintaining yourself in a horizontal position.