Go to the 'Skins' folder in the osu! root directory (e.g. 'Program Files\osu!') and enter the folder of the skin you're using. If xk.ini files aren't present, you'll need to make them. The wiki has a few notes on the files: https://osu.ppy.sh/wiki/Osu!mania#Skin
I would point you to my skin's files as an example, but I haven't updated it yet. If you have any osu!mania skins, you could check the syntax of the xk.ini files - or copy them over to your current skin and modify them how you like.
EDIT: this guide looks seriously in-depth! t/203554
I would point you to my skin's files as an example, but I haven't updated it yet. If you have any osu!mania skins, you could check the syntax of the xk.ini files - or copy them over to your current skin and modify them how you like.
EDIT: this guide looks seriously in-depth! t/203554