To program the M1-M4 buttons hold one of them and then press the Back (for single button) or Start (for combination of buttons) and the colour of the right thumbstick will change to purple. Let go of the M button and then press the buttons or button you want and then finally press the M button again. That M button will now mirror those buttons/button. I've found it very useful for using the controller in one hand whilst using the mouse in the other.
https://www.amazon.com/gp/product/B086PF8J6H/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&th=1
