Take your best shot at how you think the end product should sound (without mastering/processing plugins on the stereo bus which can skew how the actual mix sounds). This will give the mix engineer a 

better idea of the direction you are looking for and avoid them having to come up with something from scratch which can be a total time waster.

 

If you have a specific song that you like the style/sound/vibe of, provide it with the rough mix so you and your mix engineer can determine if it’s possible and how to get there. Also, be open to ideas your mix engineer has.  They get paid for their knowledge and skill and may hear something that would take your song to another level. Doesn’t mean you have to go with it, but at least consider their professional opinion before saying “no.”

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Bonus: Your mix engineer should be asking for this any way to determine what state the project is in, if there are any major issues that need to be addressed before mixing, and if the project is even something they want to work on. While rare, it’s also possible that your “rough mix” is ready to go as it is. The best engineers out there keep it 100, even if it means they don’t get your business.