Previous posts have discussed the importance of [proper buoyancy underwater]. Here I describe how to do a simple weight check in the water:
If you don't know where to begin, take about 10% of your body weight. If diving in tropical waters with a thin wetsuit, subtract 4-6 pounds; if diving in cold water with lots of exposure protection, add 4-6. This will give you a starting weight to tweak.
Enter the water
Begin at the water surface with full diving equipment and an inflated BCD.
Hold a normal breath and deflate your BCD
At this point you should find out if you are properly weighted. If you sink, you are overweighted, if you bob out of the water, you are underweighted. An ideal weight will keep you approximately eye-level.
Based on feedback from the previous step, adjust your weight accordingly and repeat until you float at eye-level.
Compensate for your cylinder
If you are doing this check with a full cylinder, you should add about 4 pounds to compensate for the end-of-dive when the cylinder will be more buoyant. Trust me, it won't be fun kicking to stay underwater during the safety stop.
That's it! Once you have a proper weight, note it in your dive log to save time in similar diving environments with the same exposure protection. Believe me, you will thank yourself two years from now when you return to similar conditions.