Hi OP, here's how I did it-
like others have said- vest underneath, any sort of top you can pull up on top. Had a floaty light big scarf thing (not breastfeeding specific).
put floaty scarf round neck- unclip nursing bra at top, put hand under 'over top', pull down vest just under boob and also nursing bra. All this is happening when your boob is totally covered with the over top. Get baby in position to feed, drape floaty scarf over baby, whip over top up, get baby feeding, remove floaty scarf from over baby to round your neck. If baby pops off you can whizz the floaty scarf over any exposed boob before anyone notices what's happened.
agree also on working out in advance what positions in the cafe/ what cafe you'll feel most comfortable in.
once I'd perfected the above, I fed anywhere and everywhere, and it's such a virtuous circle, the more you feed publicly the more comfortable you feel and the more you do it.
Good luck!
ps. Primark vests are a wonder for the job- super cheap so you can get all the colours, stretchy so easy to pull down and nice and long so don't ride up and expose post pregnant stomach..