For private commissioners, I have written two different guides, one for photo-based paintings such as portraits (of actual people) and the other for illustrations. They should answer some of your questions about my painting process, prices, what you can expect from me and vice-versa. Of course, if you have any questions, don’t hesitate to send me an e-mail!
If you are new to commissioning artists, first, that’s fantastic and I hope you’ll have great experiences with the artists you’ll work with! And second, the illustrator Randy Gallegos wrote this very nice guide about commissioning artists. Though its main focus is small presses and self-published authors interested in commissioning book covers, the info he gives is useful for everyone, so don’t hesitate to read it.