Harvest the leaves at any time but be sure to harvest them all in the fall before frost. I don't let mine bloom because it wants to go to seed after it blooms. Besides, cutting it makes it fuller with lots more leaves.
For Stevia liquid:
Cut the stems about 2/3 of the way down. This leaves room for the plant to grow more branches below where the cut. I dry mine with the leaves still on the stems. you can dry them in the sun (cover with net to keep clean) or in a dehydrator. I put mine in my hot empty greenhouse. If you use a dehydrator, set it on 95 degrees and it will take about 24 hours to dry.
Strip the leaves from stems and grind in a coffee grinder or use a motor and pestle like I did.
Some people use the leaves dried as a sweetener. It works and is healthy (300 times sweeter than sugar). I personally don't care for the taste but you should try it for yourself. .