Hmz that's a tricky one. You could wait a couple of months before letting them know you're interested, but by then it could be too late and then you'd wish you'd said something sooner.
I think you should start by being a little bit flirty and friendly but not too OTT. If they respond by flirting back then be a little bit less subtle about it and whilst they keep responding in a flirtatious manner keep going, increasing the flirtation each time you're together. Eventually it'll become obvious that you want to get back together, and by then hopefully they'll want you back too. If they respond by running away then i'd just give it some space and time and try again later if you still want to.