@KrssxXx Oooh? Best tip I can give is to keep fei in character, though my advice could differ depending on what your xReader is about. Also I'd recommend doing some research on Feitan as well. I've constantly had to check things for him and I really feel as though I understand him a lot more. Something I've noticed when writing fan fiction is that you may find yourself noticing little details in both personality and appearance the more you look at him. Also it makes writing future fits with that character way easier.
But yes, biggest piece of advice it to be true to the character. Feitan won't just go falling for anyone and if he did it would take a fair bit of time. <3