Basically the same as your original fiance visa application, plus the marriage certificate.
The FLR is awarded if you comply to the original visa conditions ie you got married.
So, if you get married, then you've a 99.99% of getting the FLR.
Please see various other threads about the downloading the application form etc.
..... but don't let it stress you. It's a simple process (apart from handing over your hard-earned money).
Enjoy the flight to the UK, enjoy the weather (! ?), enjoy the big day.
After the marriage, then look at the FLR question.... you've plenty of time in your schedule.
