You hire an artist by making an agreement with them, you commission them basically. You discuss the terms of the comission. Professional artists who have done a lot of work generally have their prices already set up e.g 2 dollars per full colour page. But that may not apply to a full comic work.
Things like contract you would have to agree between you and the author. If there are things like profits to consider then you could negotiate over price, that's if they want constant payment for their work, or you could arrange to pay them off and own all rights to the proceedings.
I don't know how payment is usually made, perhaps paypal?
That's all I know from what I've seen so far. Have you tried pitching the question to anyone random on perhaps Deviantart?
Anyways that's my 1 cent