Wash in white spirit, hang it up to dry, wire brush thoroughly and wipe dust off, wash again in clean white spirit and hang to dry again, check all links move freely and "work" those that don't, wash and dry again if required then chain lube of your choice.
Takes a couple of hours but hey-ho I'm retired so what else would I be doing?
The reason I wanted to save this one was that it was mainly only surface rust and definitely wasn't worn out as the bike only showed minimal use.
I use white spirit cos it's cheap and it washes out all the old lube etc and dries relatively quickly (and it doesn't catch fire like petrol!)