Open the top lid of your softener. It’s a big plastic tank. Pour in a bag of water softener salt. Not table salt. Table salt is for french fries, not plumbing.
Fill it about halfway. The tank comes with a little grid that holds the salt. Don’t bury it, but don’t skimp either. It needs a nice, salty bath.
Then, dump in a bottle of resin cleaner if you’re fancy. It helps keep the system from turning into a smelly swamp. Your shower will smell like nothing, which is the goal.