I used a ps4 controller but this will also work with any other kind of external device, here's what you do:
1- Connect your controller to your computer via bluetooth or cable
2- Open steam and activate big picture mode
3- Go to settings → Controller → Controller settings and make sure your controller has been identified(I recomend naming it so you can recognize it more easily)
4- Go back to Controller → Base Configurations → Desktop Configurations
5- From there you can now customize and set the buttons according to the First Cut controller scheme you are using.
If you want a more visual aid I found this video helpful:
Hope this helps, best of luck