I air dry my FB print on drying screens over the night and they get naturally curly. There are many ways to flatten FB prints, from very inexpensive to very expensive.
I have a warmpress made to mount transfer prints on fabric. They can be found kind of cheap used and they do the jobb well. It have both temperature control and timer. To be safe I have cut two pieces of acid free matboards and put the prints in between, make sure they are free from particles otherwise there will be dents pressed in the print. My press can do up to 30×40 but I never print bigger then 24×30 and I do one print at the time.
Also, only use this with well washed and dried prints. If they are not really dry the emulsion will stick to the matboard and if not properly washed chemicals will go in to the matboard and it’s no longer a clean board. Cut new if needed.
So if you have a clean and dry print, put it in the press for two minutes in 96°C and you have a nice flat FB print to mount or do whatever you do with them. Let them cool off under some weight to get that lasts bit of flatness to it.