If you grow the planet yourself, it should be fine as long as the seeds come from an active variety. I liken it to marijuana. There are dozens of varieties of the stuff, and one variety can be FAR less potent than another. Growing conditions also effect alkaloid quality in the plant.
To prepare the leaves, simplly cut them, turn them upside down, tie them together and hang them in a dry place. They will dry in a few days (or less) if your ‘bunch’ isn’t too large. DO NOT apply heat during the drying process. Room temperature is fine.
You should have success with this. The stuff is basically a weed (it’s in the ragweed subfamily) and if you give it some nice sandy soil with the right ph (don’t ask me, look it up elsewhere on the internet) then you should get a nice supply of calea zacatechichi for virtually no investment other than time.