Cost to reserve is $10/year. The process is not without pain. The FAA is currently running six weeks behind. Just because the system says a particular number is available doesn’t mean it really is. If somebody requested a number before you, and their request has not yet been processed the number will still show as being available. To find out what might be available, go here:
http://registry.faa.gov/aircraftinquiry/nnav_inquiry.aspx
Insert the two trailing letters, then use 1-999 for the range. Pick five in the order you want them, then go here:
http://aircraft.faa.gov/e.gov/NN/reserve.aspx
Scroll down and enter your five choices, then follow the instructions. Six weeks later, when the FAA tells you that all five were reserved ahead of you, start over.
If you intend to get builders insurance, you’ll need to have a reserved N-number