Doing a proper weight check is the first step. Do it with only your mask, fins, snorkel, and exposure suit. Weight yourself so that as you exhale you SLOWLY start to sink. A pocket type weight belt with individual pockets is good for this as you can quickly and easily change out weights.
Once you've done this the next step is to weigh your rig in the water with a fish or luggage scale and see how positive or negative it is. and adjust for that. And write the info down.
Then don the rig and repeat the first step. I'd suggest doing this with a tank of the type you'd most likely be using that is between 500 and 1000 psi.
Next once you think you have this close get horizontal and lay out flat on the surface. Empty your BC and exhale. You should start to slowly descend. As you descend watch your rate of that descent. Add little bursts of air if you feel it increasing too much. At predetermined depths (5ft, 10 ft, 15 ft, 20 ft) check your descent and swim around a little.
Through all this keep a careful eye on your breathing. It should remain steady and rythmic. No extra deep beaths or longer than normal exhales. While doing this do some skills like mask remove and replace. reg remove and replace. Your breathing should not change at all in anticipation of this. If it does it'll screw up your buoyancy control.
Now for the ascents. Ideally I want students to not get vertical until the last 5 -10 feet unless conditions warrant it. You have more control over descents and ascents in a horizontal position than you ever would vertical. This is why the very first skill I teach on scuba is buoyancy control and trim. We do horizontal ascents and descents and work on getting neutral in that position in the very first pool session on scuba. The whole procedure I use is laid out in my book that is available from me in print, PDf, or non audio CD with an illustration of how it's accomplished in the pool. The same could be done in open water with a slope or platform that would not disturb the environment.
Once you accept this and work on doing ascents this way they become quite easy. Especially in OW without a down line. You keep and eye on your gauges and pay close attention to your body. It will let you know if you are ascending too fast! Your ears, sinuses, and even your gear will give you indications of your ascent rate. You just need to know how to interpret them.
And for open ocean or large lake or quarry dives - learn to shoot a bag! A SMB or lift bag and spool or reel with provide a visual and tactile reference. I have one spool that is knotted at 5 intervals with 2 knots at 15 ft and 3 at 20 feet so that even without a mask or in zero vis I know when to do my stops and where I am in the water column. Make use of the tools that are out there.
And as for formal training - a course is not always the best choice if it's done by the book. By the book is actually pretty lame. Find an instructor that will go beyond the book or will do as I do and tailor a workshop to address your specific needs. A formal course with 4 students often benefits none of them as much as an individual tailored workshop will. It allows the person doing the workshop to pay close attention to you and devote all resources that way.
And since there is no card processing a workshop is often less expensive!
