Ah, well... it can be involved. Unless you have tools to remove the rear cassette just take a rag with a little bit of degreaser like wd-40 and backpedal the chain through it, clean the sprokets with the rag... don't apply the wd-40 directly on the bike it can get in past seals and ruin grease.

Once you've given everything a good wipe-down, relax, then come back and put some chain-oil on the chain and pedal it around... then wipe off any excess lube

cheers