Allright, being an artist myself who has participated in several games (sometimes commisioned, sometimes for free because I liked the project), some basic guidance from my point of view (an artist's point of view) to hook a graphics pro for your game:
1.- Paid position:
Basic rule: if you need a minimum quality for your illustrations/concepts/sprites/pixelart/voxels/UI/whatever, you have to pay for it. Artists that have spent more time painting than breathing are scarce and they are really demanded. But think of it: you are going to leverage their knowledge as they will know for sure what you need, what requirements they need to know, and reduce the time spent *A LOT* (If you talk about IK I'll know what it is. If you talk about MagicaVoxel flow I know what you mean. I will give your hints for your graphics flow based on my experience that you probably haven't thought of. I will know the differences of working with Unity or with Godot...)
Also, the difference between an amateur and a pro is noticeable at first sight; so, if you have spent so much time on your game you cannot afford not spending money on art.
If you are going to hire an artist, make very clear what you need:
# of graphics needed
Type and size of those graphics (it is not the same to create an illustration for the splash page than a 32x32 sprite).
Budget: how much you will pay for EACH concept, sprite, illustration, mesh... So # of graphics x prize of each piece = grand total of the contract. Anything out of scope will be charged, like new frames in animations, bigger sprites, etc. So you need to have a very clear vision of what you need to avoid overspending your budget.
Description of the items: is there a design doc? how are you going to describe what you need to the artist? really detailed? just a brief indication for the artist to do his best? what type of game is it? what is the game about? in what era does it happen? what kind of environment are the characters facing? etc...
Style: it helps A LOT being really clear of how do you want your graphics to look: realistic? Blizzard-like? manga? If you can provide examples of what you expect, the artist will appreciate that. Just a screenshot of Hearthstone for example and some indications: "I want this kind of illustrations but with less saturated colors" is enough.
Approval flow: most of us have a fixed workflow that has to be followed in order to avoid misalignments and misunderstundings. For example, my steps are usually these:
Requirements agreement (see the bullets above)
Contract sign off
Submission of 2-3 drafts for selection of 1 and approval (by mail)
Submission of finished piece and approval (by mail).
Payment of finished piece.
I usually grant 2 amendings to the final piece. I.E.: I may correct the final piece following your instructions for a 1st time, and then for a 2nd after submitting the first modification. After that, I will charge you for further amendings. This is to avoid constant changes of mind in the customer (sometimes it happens that it is first approved and after some time another guy in the customer's team sees the illustration and wants to change X, and then another guy wants to change Y, and then...; also some customers don't want to say "it is ok" because they don't want to pay and it is a way to say it is finished, though they actually include the image in their game).
2.- Non paid position:
As I mentioned, some times I work for free because I like the project. I say "for free" because promising money if the game makes revenues is like saying "for free". So, what do I check in order to at least take a look at your post or avoid it?:
If you tell me you need "full commitment" (very usual to read), forget about counting on me: if I join your team it will be for fun, and my kids need to have lunch everyday so probably my full commitment is in a paid position right now, and I will do your tasks when I have some spare time AND I feel like it.
This also means you cannot have tight deadlines. Milestones are for paid projects.
Be VERY clear about what you expect and the style. The last free project I attended I told from the 1st moment that I had joined because I wanted to try a concrete illustration and sprites style, to learn and try it myself. They told me I was in, to later tell me they just needed a very simple graphic style that any amateur could achieve. Thus, it wasn't fun anymore for me, the reason I joined the project didn't exist any more, and I quitted. We both wasted our time because they just wanted a qualified artist and didn't take into consideration the reasons I told I was going to work for free.
Be clear if you are going to give revenues if the game sees the light and makes money how much will the artist receive (% of sales or whatever) and how he may track those figures. Also how and when it will be paid and how the artist's name will appear in the game credits, etc.
If you are well known in the games world, I will take a look to your post for sure. If Cliffsky ("Democracy" and "Production Line" games from Positech) writes me, I will attend promptly as I both love his games and know he is a real pro and a turstworthy person.
Don't create a big hierarchy. If I am going to depend on another artist ("art director") which will also work for free and which knows less than me about art, just because he is your friend or joined earlier, it will discourage me.
If you have a HUGE number of followers I might consider working on your project because of the exposure, but it is probably the least reason for me to join.
Again, the bullets in the "Paid Position" apply, except the ones regarding the prices and probably the approval flow.
3.- How to keep an artist motivated in a project?
So, you have your artist now and have started working jointly. Everything is smooth and you want to keep it that way, so following these basic rules will keep your artis in your team:
Follow the approval workflow
Respect the contract regarding the exposure, credits in the game, etc...
Non paid position:
Answer mails, keep in touch, praise his work (yes, vanity is king among artists), expose him in your social media.
Don't talk anymore about "my project". Now it has become "our project" as there is a guy spending part of his time for free on it. This is really irritating and I find it a lot among the youngsters who think they have a big idea. In a recent case I decided to leave a project I was liking (and with me the secondary artist and the sound guy) because the leader always emphasized that it was "his project" as if we were going to steal it or something like that. Obviously "his project" never was finished even though he had some talented artists with him working for free and the Unity development was completed.
Do your work: make the game progress. If I notice you are lazy on it I will stop drawing as I won't feel motivated. How am I going to be passionate about your project if you are not?
Keep the artist updated: let him test the game after each sprint, make him aware of the next steps. If there are any changes let him know as fast as possible. Don't waste his time.