You can remove some of the guesswork out of it by checking for reward availability in First, if there is Reward Availability, you can use your SWU without being waitlisted.

Book your ticket on flights that show reward F Availability, then log into the reservation and you should be able to upgrade using your SWU.

I recently used some of my SWUs, and erroneously thought that you could use them on any flight any time... (haha). Ended up having to change dates for my outbound flight to LHR because the flight I had originally booked and was waitlisted for ended up selling out of F seats before I was cleared on the waitlist.

You can also call the elite desk and they can book for you, and and upgrade you using your SWU...