Slow and steady does it.......
Seriously, everything has to be done slow, and at Badger's comfort speed. Is the new addition male or female? Fixed? Is Badger?
Male to male is usually a ton of posturing, a bunch of bent feelings, and a longer period of adjustment. Females seems to adapt a bit easier, but it'll still take time.
Get them to meet outside of Badger's "territory" (his yard/house), a park or even a cul-de-sac will work. That way Badger doesn't have to deal with the intruder on his turf.
As far as in the house, don't let them be in the same room unsupervised for a while, until you're certain that both can be trusted. Expect that there will be some scuffles. They have to establish who's top dog (after the humans of course!) on their own, we can't decide it for them.
My Vet said that as long as they are not drawing blood on each other, let them duke it out. They sound like they're killling each other, but they're really not.
I've managed to successfully live with three males. Zeus and Apollo have been with me for over two years now, and they still have their hormonal days.
Onyx is our newbie, and he and Apollo are currently *determining* who's "king Fuzzy Butt"...