The water from YVW is as soft as it comes. short of the mild amount of carbonates, its almost as soft as Pilsen.
Without going to heavily into the hows, whys and whats, the two major salts you will ever need (95% of cases) is Calcium Chloride and/or Gypsum. Reserve carbonates (bicarb of soda is easy to work with) for dark beers (if required) or sneaky kettle doctoring of a Dortmunder export. usually add to the kettle unless the mash really needs it (check pH before debating this and/or add dark grain just before sparging/at mashout to avoid pH issues.)
First and foremost, you want calcium in your mash and boil. it assists with enzyme activity, break and yeast health. its an all round winner. Achieved simply with either Cal Chloride or Gypsum. in a 23L batch, 3g in the mash is sufficient to give around 50 ppm of Calcium for your mash needs. Scale these gram amounts based on your usual final volume (Don't be pedantic either, just round the number up).
What i typically do is add the alternate to the kettle at a 2x factor e.g. 6g to the boil for a 23L final volume.
when do i decide to use what and where? Chloride increases mouthfeel and heightens malt flavour. Think of it like adding salt for seasoning a soup, pretty bland until you add salt. I usually reserve this as a large boil addition for malt forward beers (e.g. brown ale, Irish red, Helles)
Sulfate helps hops pop and crates a refined bitterness. basically use for anything hop forward (for me its usually anything which is 40IBU+). again, add to boil and use chloride in the mash.
so what to do? don have gram scales? dont worry.
for hoppy/bitter beers, 1/2 tsp to mash fo Calcium Chloride, 1 tsp kettle Gypum
For malty beers: 1/2 tsp gypsum mash, 1 tsp kettle Calcium Chloride.
Hoppy will give you a 2:3:1 ratio~ of Calcium, Sulfate, Chloride
Malty will give you a 2:3:1 ratio~ of Calcium, Chloride, Sulfate
If you only have one of the salts, just add a pinch to the bash and a pinch to the boil (if total for mixed salts is 15g, add 10 of a Single salt addition so its concentration isn't too out of kilter.)
Happy brewing.