I think i did this right. Turns out my memory was a tad spotty but it basically covers high yield extraction with chloroform and chromatography. Probably not what the poster was looking for as i think he wants to make more of an extract of kratom (essentially a paste that contains a variety of the active alkaloids in the plant). This might be a bit too specific but you could always omit the chromatography and just do a simple chloroform extract that yields you a mostly Mitragynine compound with other alkaloids.
Kind of shooting from the hip here Check the paper.
Also, not sure if there is any interest but I have a textbook on the chemistry of Mitragynine and related compounds. Specifically it deals with the chemistry of compounds that are basically “atypical” opiates like Mitragynine. I can post that as well if anyone cares for it.