One time I did an extraction of kratom. It was a rather crude attempt, but resulted in at least somewhat effective product, as evidenced by a friend. I did not try it myself since I saw that friend use kratom a few times and decided that I'm not very interested in that substance for now.
Anyway, what I did was to cook powdered kratom leaves and filter the tea. I think I once used a dash of vinegar in the cooking part, but it seems not to be very important to do so. Then I based the tea with sodium carbonate solution. A tan precipitate formed immediately, which was filtered, washed on the filter and dried. No attempt was made to establish yield or purity.
I used vacuum filtering, so I am not sure how feasible this is with, say coffee filters, but I guess it could be done, perhaps with some patience. The procedure is very similar to extracting crude freebase harmalas from syrian rue seeds.