You don't want to melt the butter.. just soften it. I generally just leave out to get to room tempature
Take the softened butter and add whatever you want in the compound butter(herbs, spices, etc) and then mash together with a fork so its all mixed together(some people use a mixer, I do it by hand)
Then place the butter in the center of plastic wrap, wax paper, or parchment paper and roll it up in to a log
Then chill the butter until it hardens back up and then you can cut slices off your compound butter as you need them.