Totally newbie here and need guidance on how to simulate a board designed in Altium using PCB heatsinking in natural convection
Hey @hpuente ,
Thanks for posting on the forum.
The first step would be to import your CAD geometry in one of the supported file formats.
This tutorial of an electronics box cooling should be a great start to learning everything you would need to run a similar simulation on your CAD model.
Let us know if you need more support along the way! 
Best,
Satvik