To become a DPE there has to be a need as defined by the FSDO for another DPE. An ATP is nice, though not a requirement. I understand that the minimums are 2500 hours and 300 hours in the last year. The 300 hours seems to knock a lot of would be examiners out of the running.
There's a written test which (I believe) you can take whenever you'd like. You present your exam and yourself to the FSDO who then schedules you - at your own expense - to go to OK City for DPE training.
When you get back you have to sit tight until the FSDO says "yep, you'll do" and you can begin to examine. You'll likely have an FAA inspector with you for a while.
As a practical matter, it's best if the FSDO asks you, if you're an ASC, and if another examiner (or two) recommend you.
This is from an examiner buddy of mine, and he's been around a while, so it may be out of date.
-LC