Keep in mind as you hire an artist, that the relationship should remain on a professional level. This means your exchanges should focus on the art.
I've been blessed to work with three different book cover artists so far.
I've found that successful cover artists for me are
1) able to take constructive criticism (such as, 'That's good, but I need the light to be brighter,' etc.).
2) send regular updates via email. (so you can add or subtract details as needed)
3) speak and write English because it is the only language I speak. So, this last one depends on your language skills.
4) finish the cover in a reasonable amount of time. Two or three weeks - if it's a complicated cover, a month at the most.
As an indie author, concept sketches and updates are critical because you are your own business.
If you've explained/drawn a quick sketch, it should suffice to get the free-lance artist started.
In the end, you should be happy with the result.
REMEMBER You need to be professional as well. Don't keep the artist tied up while you try to decide if you want the character to have eyes or not - that is also unprofessional.