The first thing you need is good test kits for Calcium and Alkalinity (and eventually Magnesium)
Second need to understand is that you need to add both Calcium and Alkalinity and eventually some Magnesium.
Calcium and alkalinity can be added in different ways:
a) Independent individual supplements that add only one of them independently. These are mostly used when only calcium or only alkalinity need to be adjusted.
b) Two part balanced supplements: These are supplements that come in two different bottles, one bottle adds the calcium and the other adds the alkalinity. The difference with A above is that the concentrations are designed to balance the addition of both by using equal amounts from each bottle.
c) Kalkwaser (limewater) solution from a reservoir. This is the one mwethos ytou mention above. Limewater adds both Calcium and Alkalinity in a balanced proportion, dissolve two teaspoons in one gallon of RO/DI or distilled water, let it settle and slowly drip the solution in your tank. Watch your PH as limewater can increase PH if dosed too fast. Use instead of repenishing evaporated water.
d) Use either a Kalk Reactor or a Calcium reactor. These require equipement and larger initial investment. One adds Kalk like above but needs no premixin, the other also adds Calcium and Alkalinity by dissolving Calcium Carbonate.
Here is additional reference on how to select a supplementation method.
How to Select a Calcium and Alkalinity Supplementation Scheme
http://www.advancedaquarist.com/issues/feb2003/chem.htm
