You use Calcium Hardness found at any pool shop.
Recommended levels are 80-250ppm.
ALWAYS REFER TO YOUR POOL MANUFACTURER'S GUIDELINES
Fill up a bucket of water from the pool, add the required calcium to the bucket (approx. 2kg / 4.4lb at a time), mix and distribute around the pool whilst the pump is running.
150g / 5.3oz will raise the Calcium Hardness by 10ppm every 10,000 litres / 2640 gallons