Assuming you (the game designer) have located a concept artist, you need to provide them with some instructions. Fortunately most of the necessary info is already contained in the design doc, particularly the type/style of art that is going to be used in the game should be described in the features wishlist, but you will also need to find some source images to show them what you want.
Now you need to make a task schedule. This is a list of the first several concept drawings you need, in order from most important to least important. If your game is 3rd-pov the PC(s) are probably the most important thing to draw. Then the major NPCs. If a particular object, location, vehicle, weapon, monster, special effect, or animation sequence has a lot of screen time it goes here too. The gui, main game mode screens (map, combat, puzzle, dialogue, minigame), submenu screens, cursors, etc. will all need to be sketched by whoever is designing the GUI (might not be the concept artist). You will need to specify a standard image format and resolution for all concept art, and tile sizes if the game is isometric. Now give a copy of the design doc and task schedule to your concept artist, answer their questions, and don't forget to give detailed constructive criticism on each piece of concept art as you receive it from them.
For the Concept Artist
Starting from only a bunch of words and establishing the visual foundations for a whole game can be a daunting task, but it can also be a fascinating and exhilirating challenge. Here are some helpful guidelines that will hopefully make the process less confusing and frustrating for you:
1) Communicate with your designer! No matter how good your designer is, there will always be some detail you need to know that he/she didn't think to tell you. And you will have great ideas (Ooh, let's make all the animals lizard-like rather than mammals!), but don't forget to ask your designer to approve them before you waste your time making a complicated concept drawing or set of drawings that may be rejected out of hand. Check your email/the game project forums often and don't be lazy about writing emails and posts!
2) Stay Organized! Keep all emails/posts about your assignment and any documentation or other resources your designer sends you, and any source images you find for yourself. Pick a file naming convention and stick with it - I recommend an objectname_threedigitnumber_adjective.filetype system, where the adjective is a memory aid so that when you look at a whole directory full of files a month later you have some clue which file is which drawing. Similarly, keep all your sketches in a folder, and don't risk your originals, print out a copy of your lineart if you want to try adding details or colors to it. Keep copies of all your images both on your personal computer and uploaded to the internet somewhere for easy access by other project members and in case your computer dies.
3) Work in steps and revise each before proceeding. It doesn't do much good to draw cool clothes on a character whose anatomy is all wrong. Get the shapes and the attitude right first, then come back and add details or colors later. Don't be afraid to sketch several versions of something, then pick the features you like from each and combine them in a new drawing. The more important a concept is, the more times you should expect to re-draw it before you get it just right. Don't be afraid to research something you are having trouble with. Don't forget to ask others for constructive criticism, because often we are too close to our own work to to see subtle flaws in it.
4) Standardize your method to get standardized-looking results. Always use the same types of paper, pencils, ink if any, the same scanner settings, the same markers or color palette, the same file type, the same resolution for each type of concept art (you may want to use a higher resolution for color or detailed images), and so on.
I know all this sounds rather stodgy and pedantic and not at all artistic, but it really is worth it in the amount of effort and confusion it saves later.
5) Finally, be self-motivated! Even when you are working to a designer's specifications, a lot of the artistic choices and your task and time management are going to be left up to you - challenge yourself and have fun! Make sure you work on this project regularly, because it's a lot easier to put in a hour every day than to come back after a week and try to remember what you were doing last time. Remember that even if you are getting paid for your efforts, making art should be play, you should look forward to doing it. ^_^ Get yourself enthused about your tasks, and pat yourself on the back when you finish them. If you get sick of working on one thing switch to working on something else for a while until you feel refreshed. (Taking a nap often helps too. ;) ) Don't forget to stop and reevaluate regularly so you always have a clear idea of what you're doing, why you're doing it, and where you're going next. Remember that not only the other team members but everyone who plays the game is going to enjoy your art. You are turning whispy ideas into a vibrant, living, visual reality.