How to properly clean the ventilation slots on my Macbook Pro
Properly?

Head over to ifixit.com and look for your model.

Open up the bottom case, disassemble the fans. That is unscrew typically 3 screws and disconnect the cables.

Then hold on to the rotors so that they don't start spinning.

Then use a can of compressed air to blow away the biggest chunks of debris. Out of the fan-assembly, and out of the grills that would to the outside.

Afterwards use a damp cloth or better yet alcohol on a q-tip or similar implement to remove the finer and more sticky residues. 

Let any fluids evaporate.
Then re-assemble everything. 

The screwing part is easy, the cables need somewhat more delicate handling, so be careful.

Do not use a vacuum and do not let the rotors spin while cleaning!
Static electricity buildup or damage to the motors may result.

Related info:
How to deal with a loud MacBook fan. Here are some noisy fan fixes.
How to Clean Out Dust From Your MacBook or iMac
Cleaning fans, grille out of MBP