to get a Chinese visa in South Korea (in SEOUL)...how ?

I've done it.

You must have a Korean Alien Registration card with at least 6 months validity on it (so people with an ARC with less than 6 months left are ineligible). The only exception is if you have gotten a Chinese visa within the last 2 years. The 6 month validity requirement is waived, but I'm not sure if you would still need an ARC at all or not.

You MUST go through a travel agency. The consulate does not accept individuals applying. I've used SOHO travel both times. http://sohoholiday.com/ They're easily accessible in Hongdae. They speak English well and their service fee is the cheapest I've found (I think it was 10,000 won. Not more than 20,000). They'll even do it by mail (pay with bank transfer) for a few thousand won extra.

My first time, I gave an itinerary and all that stuff. The second time , I didn't. The travel agent (for getting my visa, I did all my own booking) said it wasn't necessary. No itinerary, no hotel reservations. I can't remember what I did about flight information (I flew in and out of Hong Kong). I'm pretty sure I told them my expected travel dates. Every night I stayed in a hotel was a walk-in.

I believe the visa processing time was 5 business days.