Id second this recommendation preference. Finding good designers is hard. Id say generally;
1) Don't go by a persons confidence to do the job. I've often found confidence in ability and quality are inversely related when it comes to designers.
2) Try to find someone that understands the business need and not an artist. Not that art is bad but I find the artist type risks getting too involved in their work. You want someone that isn't too emotionally invested in the work where they cant take genuine feedback or scrap it and genuinely try again with fresh take. There's a reason agencies put account managers between creatives and client.
3) Good design takes time. It can seem like a long time to get to something simple but its usually needed.
4) Don't committee things. You can polish the diamond but when you've got a bunch of people expressing design ideas to be merged together it tends not to work. Also if there are multiple people commenting, have someone on your side take that commentary and centralise it to the designer, dont drip feed them comments or let a bunch of people all send their own thoughts.
5) If its not working dont try to make it work. Some designers are really good but just dont work well with the vision you want. Its not their style. If its not working it probably wont.
6) Be specific in your instruction. Don't give them the 'I want it to look like example A but with touches of example B' type deal. When people look at the same thing the see different elements as what makes that example that example. Figure out why you like something, make sure that is the reason on example 2. Really consider the language you use when writing any design requests if you have something in mind.
.... a bit off the cuff but hopefully that useful. Good luck.