How do I do this? cuz I sorta enjoy the idea of sleeping with Jaheria.

The romances are sets of dialogue which automatically occur with the potential romance candidates whilst simply travelling about in non-dungeon locations.
New dialogue occurs about every hour (real time not game time) and some romances have additional quest aspects to them (especially Jaheira's).
If you want to do Jaheira's romance make sure that you download the latest patch as it's very buggy.

<font color = "lightgreen">Try a search for posts about the Jaheira romance; there are plenty of people who have pursued the lovely half-elf.</font>