A commission is the same painting process as anything else in the gallery. The only difference is that the subject is yours, and you have a picture in your head before I start. Most of the work is making sure the picture in my head matches it.
Here is the whole of it, plainly.
It begins with photographs
The single thing that decides how good a commissioned painting can be is the reference photograph. Not the camera, and not the resolution. The light.
A photograph taken in flat overcast light gives me very little to work with, because there are no shadows telling me the shape of anything. A photograph taken in low sun, early or late, gives me everything. Send several if you have them, from different angles, and do not worry about composing them. I would rather have six honest snapshots than one careful one.
For a house or a vehicle, one photograph from the angle you actually want, plus a couple of detail shots, is usually plenty.

That one is on the site now, and it is also one I am happy to paint again at a different size, which is a thing worth knowing. Some pieces can be commissioned as a version of themselves.
Then size, and it matters more than people expect
Small is not simply large made smaller. At 5 × 7 I have to leave things out, and choosing what to leave out is most of the painting. At 14 × 12 I can put in the detail on a grille or the individual boards on a wall.
So the size conversation is really a conversation about how much detail you want. I will say honestly if a subject is too busy for the size you have in mind.
What pastel does well, and what it does not
It does light, weather, fur, rust, old paint, water and sky extremely well. Anything with a soft edge and colour sitting inside colour.
It is less suited to hard mechanical precision, very fine lettering, or anything that needs a ruled line. Not impossible, but it is working against the medium rather than with it, and you can feel that in the finished piece. If what you want is essentially a technical drawing, I will tell you so rather than take the commission.
Faces I will discuss case by case. A pet is straightforward. A person is a different undertaking and I would rather be asked directly.
The part nobody mentions
There is no undo in pastel. That is true of a commission as much as anything else, which means I work out the composition before I start rather than discovering it on the paper.
In practice that means I will come back to you once with a plan, in words, before any pigment goes down: what is in the frame, what the light is doing, what size. That is the moment to change your mind. After that the painting has to run its course.
Timing
It depends on size and on what else is on the easel, and I will give you a real answer when you ask rather than a hopeful one. If it is for a particular date, say so at the start and I will tell you plainly whether it can be done.
If you have something in mind, the shop has the commission listings, and the gallery will give you a sense of what the medium does. Worth reading the framing notes too, since a commission arrives unframed like everything else.
— DawnE