First of all, decide the type of painting you would like to commission: baby portrait, body parts, abstract, rangoli, motherhood, pregnancy, in oil colors on canvas or pencil/colored pencils on paper.

For baby portraits it is necessary to choose a photograph that will serve as a reference for the final painting. The picture doesn't require a high-quality camera. A phone camera will suffice. 

For other types of paintings, it is sufficient to have some ideas or references in your mind, paper, or digitally.

Contact the artist or directly via email for your commission.   

The artist will inquire about more details like size of the canvas, place you would like to hang it, which is important for the color pallet. He might need more references for the portrait. 

During the process, the artist will email you photos of the work at a particular stage and especially before the final strokes for approval.