Without a fairly high end setup including a rotovap and the like, your best bet is to go for a simple full spectrum resin extract using alcohol/water and a touch of h2o2. Saturate powder with H2O2 first, let it sit for awhile while it attacks the cell walls, then soak in ethanol. Everclear if it's legal where you are. The second wash, hit it with just enough water to saturate it again then filter it out with cheese cloth or a t-shirt, making sure to wring out all the water you can. Run your liquid through a coffee filter to catch whatever remaining sediment. Evaporate it off to resin.
If potency is your goal, you can further oxidize a good portion of the mitragynine to 7-oh-mitragynine using 10% h202 and UV-A/B/C radiation. Sunlight works, directed UV bulbs work better though unless you're in a heavy UV zone like an Arizona summer. Dilute 35%H2O2 to 10%, add enough to your kratom resin to fully saturate it, then spread it thin on a pyrex baking dish and set it under a UV bulb for reptiles/reef tanks or in very strong sunlight for a day while it dries back out. It won't oxidize all of the mitragynine, but it does a great job at converting most of it.
This is a good way of doing it to have on hand.
Anecdotally, with no analytics behind it, just a lot of internet discourse and a process I've tried out with success myself:
You can also start a fermentation process on leaf powder then kill the culture with a splash of 3% H2O2 after a day of letting it roll. This sparks an oxidation process in the tea that also converts a solid amount of the mitragynine. But this is really only good for a day's worth of dosing at a time. With oxidizing the resin, you've got something high potency you can have on hand to use alone or stretch kratom leaf further.
Without growth, what's the point?
Avid gardener and Ethnobotany enthusiast