How do you apply to offer WINGS credit for an event you are sponsoring?
A fly-in by itself doesn't usually qualify for WINGS credit, but if you're also running educational programs or seminars there's a good chance you've got something that will qualify.  

Normally you'd work with a FAA Safety Team representative to help set up the program so it meets the WINGS credit requirements (they have access to premade presentations, slides/videos, and with a little notice they can usually help find presenters/speakers), and the representative will then coordinate with your regional FAA Safety Team manager to get the programs listed in SPANS so people can sign up and receive credit.

If you have an account on faasafety.gov you can search the FAASTeam directory for representatives near you:

(I did a cursory search and I didn't see anyone in Douglas, but there are a few folks in Tuscon.)

If you can't get in touch with any of the representatives your best bet is to contact your local FSDO (Scottsdale in your case) directly and ask for the "FAA Safety Team Program Manager" - they'll be able to work with you to get the program coordinated and set up in SPANS.  



If you intend to do regular events with safety programs it would probably be worth getting yourself or someone else in your group set up as a FAASTeam representative. There's some (really simple/basic) training involved, and usually the regional program manager or one of the lead representatives in your area will want to meet with you just so they know what you look like, and once you're set up you'll be able to submit requests to have your programs listed directly.