Model 1 Megadrive:

It is recommended to do on a Model 1 Megadrive, due to having much better documentation of the board. This mod puts your console in NTSC mode, but due to the oscillator it will run at 59.3 vs 59.94. The difference is incredibly small, so this is allowed on the board. This mod will set your console to NTSC, if you are looking to add a switch to swap between PAL and NTSC (or load certain region locked games such as Sonic Classics cart) there are plenty of easy guides to follow, however since it is not needed to use a PAL system on this board it will not be posted here.

Edit: Model 2 Megadrive (follow at own discretion since Model 2 boards vary more)

https://www.instructables.com/Sega-Genesis-Model-2-Region-Mod/