You can either add to top off water or just mix it in a glass of water. (get powered calcium, liquid calcium won't last very long, you get way more for a better price) Pretty much any brand of calcium is fine, i like Seachem Advantage Calcium because it contains a few trace elements as well. (Kent Turbo Calcium is a good product too). The directions on the product should tell you how much to add for how many gallons to raise calcium so many parts per million. Mix it vigorously let it sit, pour into sump or add directly to the tank but don't pour anything that settled at the bottom of the glass.
Oh and if you're adding a product to bring up alk, don't do it at the same time, wait a few hours but preferably 24 hours.