While you cannot choose a seat, they board Business Class passengers first. You will wait in the Metropolitan Lounge, which is very nice, and they will announce when your train is boarding. They will tell you which track to walk to (they are labeled).
Then, you walk to the train itself, and when you see the first conductor, just let them know you're in Business Class. Generally, Business Class is on the front on the Wolverine, so they'll have you walk all the way down to the front of the train. There should be another conductor positioned near the front of the train, and they will assist you as well.
The Wolverine does not sell more seats than it has, so you are guaranteed a seat in Business Class. Here is a picture of the Business Class car:
(from FlyerTalk user Seat2A)
I always take a single seat if available so couples/groups can sit together in the double seats. The seats are really comfortable, with leg rests and foot rests. The seats are about 58 cm wide and have trays and cup holders. There is also a strip of outlets along the wall, under the window. Each seat pair gets one outlet.
On a rare occasion, they use an older Business Class car that doesn't have outlets, so be sure to charge your phone and laptop before boarding. I've only encountered this once out of many trips, though.