Reply to help with setting up a "shift" button

I got a launchpad and want to make a shift button. I know it's to do with modifiers but I'm a bit stuck. I tried to do it but just couldn't figure it out.
I know it doesn't make a difference at the fact its a launchpad, any controller is the same principle. But how is it done? I got 3 buttons for example, and 2 of them do a certain thing but when I hold do the 3rd button it acts as a shift button for the other 2 to make them do something else. Can someone talk me through how to do that please.

Much obliged.