Discuss pay directly, address your budget, or your model of payment ASAP, whether you need a volunteer slot filled, paying per piece, per hour, budget, rev-share, etc. Also discuss things such as ownership of the assets, in some countries and some states emails can be used as legal evidence in court, you may want to look into a contract specifying everything to protect you and the artist.
I can only answer bits and pieces of your questions based on my experiences, but as a coder/designer to make the artists job easier, learn how to use art in any format they send, for example if they are making you 2D sprites, you will probably be receiving sprite sheets, learn how to import sprite sheets in whatever engine you are using, so that you don't have to look like an idiot and ask the artist to export every still image seperately as well as sending you animated versions for actions. Learn everything on your end, learn basic photo editing incase you need to make minor adjustments, be as clear and detailed as possible about what you want/need so you don't need many (if any) revisions from the artist. I personally like to give artists the freedom to make their own ideas come to life, i'll give a brief prompt of what i want, size, format, etc, theme, color pallet, but i don't want to micro-manage and get too particular about my vision, i want the artist to do what artists do, and make art.
Make their job as easy as possible by either knowing exactly what you want, sketching up concepts to show them so they can understand what you want, or keep it open ended and trust them to make what you want, either method can work.