Beware, this process is longer and more complicated than it used to be. The online application says to allow for 6 months - I would add extra time just to be safe.
(NB: this is for the first passport only, replacements passports are still straightforward and take 4 - 6 weeks).
The process starts with the online application which has been standard for a while. I submitted the documents requested at VFS Global in Phnom Penh tower, payment forms etc.
Weeks later I receive a letter by email (attached as pdf) requesting further documentation, all requiring translation into English. These were:
1. Immunization records of the applicant
2. Full ante-natal medical records and birth medical records
3. Family photographs from marriage to birth to after birth of applicant
4. Details of any other children who hold UK passports
5. Cambodian identity card held by mother and father (pretty sure we included these in the original application)
Once you've got all these, you'll then need to arrange another appointment at VFS to have the documents checked and sent to the UK (fortunately, they don't charge you for sending these).
After some time you'll then get another letter telling you to call a UK number to arrange an interview. This interview might be a month out from the date you call. The interview is held in VFS via Skype to the UK.
Then, a few weeks after that, the passport arrives.
