First of all, there is no such things as an 'FLR' visa. ALthough your visa may have FLR on it, so does every single other visa issued from within the UK, from Student to Investor, so using this as a description of a visa just makes things REALLY confusing!!!
The route is as follows:
Get fiancee visa
Come to the UK and marry within 6 months
Switch as soon as possible to spousal visa, which will be granted for 2 years
Two years later, no more than 28 days before the spousal visa expires, apply for Indefinite Leave to Remain
If you wish to, you can then apply for citizenship after you have been here for a total of three years, but you don't have to. You will have ILR for the rest of your life, even if the marriage breaks up, as long as you don't leave the country for 2 years without returning.
Victoria