First step: acquisition.
You will be given a quest called "Take a Break" from Dane which will take you to what appears to be a seemingly normal rooftop. The target circle will be spread out, but your goal is to simply find an armchair to sit upon and relax. Surprise! That armchair is far from ordinary; it is the Armchair-a-geddon, one of the Sin weapons (Sloth).
Second / final step: slaughter.
The Armchair-a-geddon comes with two weaponry modes: a missile launcher with
and a rotary cannon with
. One of the challenges that you will be given will require you to actually kill demons using the missile launcher, so you might want to upgrade that (as it does reach rather far and higher than your bullets will). Simply locate a good place to go on a killing spree, perhaps a Marshalling Ground before you convert the control stones, and unleash the beast... but without putting too much effort into it.