https://fly.rj.com/MileageCalculator/

A simple shorthall (e.g. JFK-BOS or LHR-MUC) return in economy will cost 20.7k miles.

AFAIK you will need to contact RJ, either through the redemption form after logging into your Royal Plus account, or by contacting them over the phone/in person.

http://www.rj.com/en/ffp_redeem_miles.html?learn_more