There isn't much to it. For a thorough treatment of the subject download
http://wetnewf.org/pdfs/Brewing_articles/BT_Alkalinity_II/AlkalinityPtII.pdf. If you don't want to be bothered with all that there are a couple of essential points that many prescriptions miss.
1. The process will usually reduce the smaller of the alkalinity or calcium hardness to 1 mEq/L (50 ppm as CaCO3). Thus if you have water with 2.5 mEq/L alkalinity (125 ppm as CaCO3) and hardness of 2 mEq/L (100 ppm as CaCO3, 40 mg/L as the ion) the hardness will go down to 1 mEq per liter.
2. The larger will be reduced by the same amount as the smaller. In the example in 1) the hardness was reduced by 1 mEq/L and so, thus will the alkalinity be: 2.5 - 1 = 1.5.
3. Because of the above an obvious trick is to increase the hardness if it is less than the alkalinity. In the example here increasing the calcium to 2.5 mEq/L will result in both alkalinity and hardness being reduced to 1 mEq/L.
4. You may well want more than 1 mEq/L (20 mg/L). If so add that into your calculations. If you wanted 2.5 after the boil understand that the alkalinity will be reduced from 2.5 to 1 with calcium supplementation implying a reduction of 1.5 in the hardness as well. If you want 2.5 mEq/L calcium after the boil and have, in this example, 2 to start with and will lose 1.5 leaving 0.5 then you need to add 2 before the boil. Check: You originally have 2.0. You add 2 giving 4. You drop 1.5 and thus have 2.5 left.
5. Add a little precipitated chalk to the water before boiling in order to provide nucleation sites and thus promote precipitation
6. If you can, raise the pH of the water by adding a bit of some base. Ca(OH)2 is ideal for this.
You can, of course, save yourself the trouble (and risk) of boiling by using just Ca(OH)2. All the enumerated tips still apply.