First Year: Plant (or buy) in spring, grow all year. Take them inside for the winter: Taste the leaves & mark your strongest in sweetness plants. If any are very week in sweetness mark them also.
Second year: Put the plants back outside in the spring as soon as possible (but protect from frost). After a week or two, taste the leaves again. Take cuttings of the ’strongest’ (in sweetness and growth) and plant them in new pots to grow. Treat these cuttings as year 1 plants. Harvest the strong sweetness plants (completely): Dry and use like tea leave (or do other crazy stuff to get the white powder form). Let the less than sweet plants go to seed, and save the seeds for the following year.
Stevia will grow for several years when protected from frost. The second year is almost always the best year for sweetness level & taste. The “sweetness” of the leaves does not seem to be passed from parent to child by seed. The seeds of a very sweet plant may produce bland leaves or vice versa. Plants grown by cuttings do seem (for the most part) to hold true on the level of sweetness.
Do save seed and plant some from seed each year… otherwise your gene pool will be very poor over many years from just cuttings.
Also during the year, leaves can be pulled at any time, and used like mint leaves to sweet teas and drinks during brewing.