Thought I'd post about the Korean National Police Certificate (required for packet 3 if the intended immigrant is Korean or has lived in Korea for certain amounts of time), as I've not seen much detailed information about it.
Korean citizens: According to a poster that I saw when I went to obtain my certificate today, it is now possible to apply online for the Korea National Police Certificate here. Unfortunately it seems that the online service is currently only available for Korean citizens; the site did not recognise my ARC number. Still, useful if the intended immigrant is Korean.
Foreigners residing in Korea: The process of obtaining a police certificate at a police station is laid out in the packet 3 instructions, and are mostly on the money. A list of national police stations is available on the KNPA website. A few things from my experience obtaining a certificate today:
1) It may be necessary to make it clear that the certificate is to be used for a visa application. The certificate I obtained today states that it is for "personal verification only" (본인확인용) and goes on to list the penalties for using the certificate for any other purpose. From what I gather, the old application form for the certificate had a box (내용확인용), "for confirmation") to check for this, but it was not present on the form I filled in today. I'll be popping down to the Jongno police station tomorrow to get this confirmed and cleared up, and will update on how that goes. (I've read that they're well-versed in what's required for immigration, probably due to their close proximity to the US embassy.)
2) Otherwise, the form for applying for the certificate is straightforward and is in both English and Korean. There is a line with 3 boxes for stating the required scope of the certificate: check all 3 for criminal records, investigation records, and lapsed criminal sentences.
3) It may be a good idea to bring both your ARC and passport. I was asked to present both (although this may have had something to do with the photo on my ARC being almost unrecognisable as it has gradually faded and disappeared over the past 5 years).
4) The packet 3 instructions seem to be correct about the check taking less than 24 hours; mine was done while I waited today at a pretty central police station in Seoul. (I suppose it might take a little longer at a busier police station such as Jongno, though?)