We used it on the chain as well as the other metal parts, and it worked well; however, after discussing with some bike experts and receiving comments about it, it has been determined that using it on the chain is a little risky. If you properly clean off the acid (by rinsing and drying thoroughly), it should be fine, but remnant acid could weaken the chain. So be careful!
Step 1: Materials
Citric acid: We used some lime juice we had leftover in the fridge, but any citrus-y juice will work--lemon, lime, etc. According to Urawaza, the citric acid reacts with the iron oxide (rust) so it can be removed.
Abrasive: We used steel wool, but anything gritty to help remove the rust will do.
Paper towels or an old rag. To clean.
A bike. Trying to remove the rust from a non-existent bicycle could prove too much for this Instructable.