Hi,
You're going to get different answers on this, but I started when I changed the lights to 12/12. Just added 1 tsp to gallon of water, PHed it, then watered plants with it, every other watering.
After 4 weeks of flowering, I added 2tsp per gallon and continued watering, every other watering.
Its suppose to add simple sugars, which will increase resin and bud production! (I think)
Some people say don't use it, but I did! First grow, so I can't compare it to NOT using it!