Dave,
Each MOC (member of congress) has an application that you'll fill out, and most have different deadlines. For example, my congressman's application for a nomination was due November 15th, by my senators both had their due dates November 30th. They will contact you if they perform interviews: only my congressman interviewed me b/c the senators in my state don't do interviews period. Other states have MOCs who select people to interview, based on potential from the application.
Hope that answers your question!