Well I'm a back-row with massive aspirations of playing as a back (which have been shot down frequently :lol: ) but I can probably relay advice I've been given.
You should probably give a more detailed break down of your skill set/weight/height/speed etc but
VERY GENERALLY:
For Back Three:
-Positioning, you need to know where you have to be in both defence and attack, if you're watching a game just try and keep note where the pros stand.
-Work on your high ball, you need to be extremely composed under the highball, and even as a centre a high one shouldn't phase you.
-At full-back, you really have to be able to kick from hand. Work on making touch, it doesn't have to go half the pitch, just make sure you can
reliably make touch.
-Learning to come off your wing/from deep at full-back is an increasingly common skill and will definitely get you noticed. Choose your line and run it at full pace with good timing (very much easier said then done)
Centre:
-Tackling is an absolute must. Can't afford to miss anything in the centres.
-Distribution, probably also necessary if you want to play full-back, passing accurately from both hands under pressure.
That's pretty much the most general advice I can give. I'm sure a back could give a lot more helpful advice but hopefully that's kinda helpful (oh and plus rep?

)