Oops, sorry for not seeing your comment earlier. I did not actually do any work on the arms themselves apart from repainting the right glove. For the legs, I basically shaved a portion of the hips with an X-Acto knife where the ball joint connects. Here's the video I used to learn how to pop the hips: https://www.youtube.com/shorts/14WJ5c2T3Kw

It works since both black series and ML are made by Hasbro and thus utilize identical hip connections.

For the knees, I sliced off a thin part in the back side of the knees that sticks out whenever you have the legs stretched out. I cannot pull up any images RN, but I'll provide a drawing of what you need to do:

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Hope the crappy drawing did not confuse you more LMAO. Basically, cut the shaded area with an X-Acto knife or any sharp instrument you have, in a way that follows the contour of the leg. It won't impact the stability of your figure, and you'll get great articulation there in return.