Depends on your skill level and experience.
For me, easy. Cutting a hole in the proper location for the vent and adding duct-work,I have done plenty.
BUT WAIT! The really hard thing is, splicing into the existing ducting will steal air from the existing ducting. The proper way is testing your HVAC system and doing calculations to see if it is feasible.
Regno, Regnavi, Sum sine regno, Regnabo