Editing or adding instruments?

I like the score aspect of Musescore and think it's pretty intuitive. I've been searching for how to add or modify an instrument and not coming up with much. I've seen Pedal Steel Guitar here & there and even made an experimental score a couple of years ago with tablature. I really don't know how I got that far. The problem is my pedal steel is 12 string (not 10) and a cutom tuning I'd need to configure. I'm not concerned with adding pedals and levers at this point but can someone point me in the direction of how to customize the number of strings and tuning so I can do some tab with my score?