One tsp CaCl2 (whatever the LHBS sells which is probably a mixture of the dihyrate, monohydrate and anhydride) per 5 gallons of water. 2% of the grist should be sauermalz (acidulated malt). This is a recommended starting point i.e. you should brew the beer this way the first time. The second time, if you are interested in a more British quality, add some gypsum but do gradually e.g. a half tsp/5 gal in the second brew etc.. You can be roughly guided as to how much to use by trying some gypsum added directly to the first beer to taste.
The salts go into the water. The sauermalz goes into the grist.