You load up your cation exchange resin with sodium cations by passing a sodium chloride solution through it. Then by putting the house feed water through the resin, you exchange the calcium ions in the feed water for sodium ions. You will need a test kit to find out how often to regenerate the resin with sodium chloride. Another method is to use a reverse osmosis membrane system. This will likely be more expensive but need less maintenance. This one works by not allowing calcium ions through the RO membrane.