STEP ONE
Go to "Manage ...(currently" on your managers tab

STEP TWO
Then go to the FPL tab and click transactions

STEP THREE
Finally go to the left most column and that will be Select Rosters from where you can select and finalise your roster.

LASTLY, REMEMBER TO EXPORT AS YOUR FPL TEAM. IT IS A SEPARATE EXPORT FROM YOUR BBA TEAM.
YOU NEED TO EXPORT FOR BOTH YOUR BBA TEAM AND YOUR FPL TEAM