When you ult someone with Morde, you and them both get taken to a different world. Here, it is just a 1v1 with no interruptions for 7 seconds. Use both Ignite and Exhaust to make sure they don't get away. Then, use Everfrost to hit them, gaining either a slow or a root. If need be, use E and Q to deal a ton of damage. If not, walk at them menacingly and auto them 3 times, then let your passive finish the job for extra time in the ult.

In the earlier stages, because SSM is mostly about BM (Please play this in a 5 stack or something) you want to mainly just focus on CS. Ganking a Morde at level 6 when he's farming under tower is near impossible, so just focus on hitting your items early. (Hence why I take Future's Market.)

By now you should be looking for maximum value ults. If you're going to ult, make sure you don't ult someone who has QSS, as they can use it to get out of your ult. Typically people like Lux, Soraka, Miss Fortune, and Jhin are generally fine people to ult. (Make sure you don't ult a super fed 20/0 though.)