It all begins with their website http://www.usaid.gov/careers/.
At the website, you can get all sorts of information about what the Foreign Service is, what the different positions are (called "Backstops" in USAID lingo), and what vacancies they have. After that, it's as simple as applying like any other job. The initial application is a bit more intense than some other jobs in that you'll need to write several short answer responses to questions on your experience as it relates to various key skills of the position you are applying to. Just note, pretty much all positions have a requirement for a graduate degree.
Afterward, if you are selected, you'll be contacted for an interview in Washington DC for their Oral Assessment.
USAID Oral Assessment
The interview or "Oral Assessment" is similar to that of the State Department. There will be 3 components for your interview which will include a Case Study (write a memo or response to a situation relevant to the position for which you applied), a Group Exercise (discuss and develop a solution to a case study with the other candidates), and then Structured Interview (same as typical interview) with a panel consisting of a HR type, a FSO, and a Technical expert in your position. After the day is complete, you will be finished and you can expect to receive a response on whether you will be accepted from 2 weeks to a month. They will follow up with references that you provide and your package of your scoring from the interview, your experience, and your reference responses will be forwarded to a final panel for determination (Keep in mind this is partially made from assumptions combined with my experience. Actual internal process may differ).
Security/Medical Clearance Process
If you are successful, you will begin the Pre-employment process. This consists of obtaining a Security Clearance and Medical Clearance. Lots of information will be provided on the specifics of this process but expect it to take several months at least to obtain these. Security Clearance should be ok so long as you have good credit, have a fairly clean background, and have kept a good record of places you have lived/worked and can have different references vouch/confirm.
Medical has many conditions but you'll probably be able to obtain clearance if you could probably live in a isolated country with limited to no medical facilities available. USAID positions are often deemed as very "Technical" positions so a chance for a limited medical clearance may be possible if your position is in a "critical" or priority backstop.
Final Suitability Review/Position Availability
Once these hurdles are cleared, you will be on their register or list of eligible candidates. Cleared candidates are grouped by their backstop and then often rank-ordered. A panel will then review the eligible candidates and make their selection on those to appoint. For the past two years, every other month or so would be an orientation class where they hire new batches of candidates. In a conversation with the DLI hiring coordinator, he mentioned they are trying to keep a good queue of a couple hundred people to select and hire from.
General advice through this process is to be prepared with all the information that is expected of you in regards to the Security Clearance and Medical Clearance. For Security, have lists of different references for all the places you lived/worked and a supplemental list of people they can contact and meet with. For Medical, if you have any conditions, be sure to get a written opinion from your physicians and send that all with your packet. Finally, patience is important because as this is a federal government position, the whims of politicians, budgets, and other circumstances can all affect hiring. Keep in touch with your HR contacts and you'll hopefully make it through. Also, a forum where many people discuss the USAID Foreign Service is found here: http://federalsoup.federaldaily.com/forum_posts.asp?TID=4834&PN=1&title=usaid-junior-officer
My Candidacy Timeline:
Submitted Application online
Referral List Issued – Referred to Selecting Official
Invitation to Interview
Interview at USAID.
Request to submit updated references
All references submitted, application sent to USAID HR.
Security Clearance (EQIP) request submitted.
Temporary Security Clearance Granted
Medical clearance form submitted.
Personal interview for Security Clearance.
Medical clearance received.
Received Salary Offer.
Full TS Clearance Granted
Appointment received to DLI 16 beginning January 2011. Requested Deferral - Granted.
Appointment received to DLI 17 beginning in March 2011.
*Disclaimer: If any of this violates the Non-disclosure Agreement, please let me know and I will remove. To the best of my knowledge, the posting contains general information readily available at the USAID website or is generic in nature for disclosure*