It was done on the stove top and not in the oven, that would cook it more which isn't what you want, normally I use a large pot and inside place either a veggie steamer pan from a rice cooker or a steamer pot (just a pot with holes in the bottom), remove the handles and place into the big pot upside down.
Over a low flame with an inch or two of water, check on it to make sure the steam is flowing but not boiling the water, a slight simmer is fine though.
Let it go for 2 hours then check it by stabbing it, that will give you an idea of how close you are.
You want it to be very soft, 2-3 hours is usually good, if it goes longer that won't hurt, the ones in the restaurant steam for hours and hours until they get used, mostly it put moisture in the meat and makes it more tender.
I have tried it with pastrami that I cut a slice off of and it still works to steam it but doesn't really hold the juices in as well since they tend to run out of the cut section so I would say it's best to leave it whole and then steam.
Also if you bought a store bought corned beef be sure to give it a good long soak to remove the excess sodium, otherwise it will be really salty.
Do a fry pan test before smoking it to make sure it tastes right.
Good luck, any other questions feel free to ask or just send me a pm and I'd be happy to help.