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.
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