A:
There was a good posting on Google as to the harvesting and preserving stevia.
The plants are usually harvested in late fall before the first killing frost. The cool temperature and short days will intensify the sweetness of the leaves.
Cut the branches with pruning shears, strip off the leaves and the stem tips. If you are in a frost free area, the plant may survive the winter if not cut too short. The second year's harvest is said to better than the first year.
Dry the leaves quickly in the sun for about 12 hours or inside in a location with good air circulation.
Crush the dry leaves for storage or make an extract using 1 cup warm water to 1/4 cup of freshly crushed leaves. Let it sit for 24 hours and then refrigerate it for storage.
It is good to hear that you had success. Stevia is a little hard to grow.