are you giving her a ride? if so, an easy way to do this would be to text her telling her to be ready by 6 or whatever. short and simple. it's only damaging if it's done days in advance and is unnecessarily long. keep it blunt. always.
if you're meeting here somewhere, that's a little bit harder. sometimes it's okay to be up front depending on the vibe she's given you beforehand. this is the method i prefer. a safer alternative would be to suggest a change of venue. i.e. i've got this other place i wanna show you. (restaurant name) @ 6.
by the way, the upfront method would just be to text something like, ""restaurant name" at 6. no question mark or anything, don't even imply the possibility of her flaking. cos if you do, she might capitalize on the opportunity and leave you hanging. remember, keep it blunt so you don't come off ass needy/obsessive.
in most cases she'll reply. if not, then don't go. if she makes a stink about it, say you assumed she didn't reply b/c something came up and so you made other plans. then reschedule.