Move the tactic file (should have extension .fmf) to ‘tactics’ folder.... which can be found through the following directory:
Documents > Sports Interactive > Football Manager 2020 > tactics
Create the tactics folder if for some reason it does not exist.
The above directory changes to ‘Football Manager Touch 2020’, if using Touch.
In-game, go to tactics page. Where you select your tactics (theee options to the top of formation graphic), ‘Load’ tactic, and it should be in the directory that pops up.