How should I sand before the final coat of polyurethane?
Depends on the quality of the existing finish... if it's as flat as you want it to be, then I'd kiss it with 150 on a pole sander. 


Link for illustration purposes only: there are many out there...

If you need to knock down blobs/ runs/ etc, then start with 120 on the handheld random orbit sander. Work your way up to 150/220. 

Vacuum and then wipe with a compatible tack cloth to remove dust and residue.

If you're having a hard time laying down a perfect coat, make the last one a bit thin -- 10-20% added paint thinner -- and it'll flow better. Also, don't skimp on the brush quality.