^^agree. Remove rear wheel. Break chain. Unbolt derailleur cable anchor. Remove rear derailleur. Disassemble pulley cage assembly. Clean entire derailleur. Grease pulley sleeves. Reassemble derailleur. Lube all pivot points on derailleur, all threaded parts. Exercise. Wipe off excess.
Degrease and lubricate chain while it's off the bike.
Reassemble bike.
This is good to do every year of riding, or so, anyway.