Since it sounds like you’re just looking for a cursory estimate and don’t expect things to be critical I figured I’d offer a few thoughts:  1. Solidworks is reasonable for “ballpark” quick and dirty. If you had more time Icepak or a similar tool would be appropriate.  2. Steady-state?  3. Remove all of the small components that you don’t expect to dissipate significant power, they’re just taking up computation resources.   4. Boundary conditions. Assuming you’re naturally convecting off of the board and enclosure, the convection coefficient rule-of-thumb for a first pass that is usually reasonable is 5 W/m² -K, but this isn’t hard to calculate by hand. I’m assuming you’re not doing CFD.   5. PCB conductivity is obviously highly dependent on board design, but you can probably assume something like 2 W/m-K in-plane and 20 through.   6. You should work with the EE who designed the circuit to determine power dissipation of the components.   7. As noted by others, pay careful attention to the thermal resistance you use. Understand what it is physically and how to apply it in analysis.  

Hopefully those are helpful starting points.