It's actually no different than if you didn't romance someone in ME2 (though some of the dialogue does change). Stick to Paragon dialogue options when talking to Ashley on Mars, and during the 3 hospital visits. Being open and apologetic about the romance with Tali doesn't hurt (though Ash does take it better than she does with Miranda or Jack).
Eventually, you'll get an email from Ashley to meet her at the refugee dock on the Citadel. Be friendly and supportive here, and she eventually emails you again, asking you to meet her at Apollo's Cafe in the Presidium Commons (she'll be marked on the Citadel map). When Ashley brings the topic of your relationship up, respond positively to "lock in" Ashley as your LI. I'd suggest you be careful how you talk to the other romanceables too, or you might find yourself locked into a romance with someone else (thus locking out Ashley's romance). No love triangles this time around... Terminator-HIX
^Catty dialogue if you bring Ashley on the dreadnought, though. Funny stuff.
^ Bringing Liara isn't bad either, if you romanced her in ME1 ;D Nothing like the "love triangle" confrontation between Liara and Ashley/Kaidan in ME1, though. Terminator-HIX