Accepting commissions is something most artists do throughout their careers. How much of your career rests on it depends on how well you do them and how much you enjoy doing them. It helps to have a specialty. If you love cats and prefer doing cat portraits, are particularly fond of Siamese and constantly go to cat shows, the best career for you might be to start painting the champions at the shows and sell portraits to breeders and serious cat fancy regulars.
Advertising can help you get commissions. So can word of mouth and online presence. In fact, whether you planned for commissions to be part of your career or not, odds are at some point once you draw well people will begin offering them.
This is because it's a thrill to people who can't draw to be able to decide what the painting will be. A portrait of someone they love, a favorite scene, anything that they wish they could draw and can't is going to appeal tremendously to people. Or they saw something you did they liked and it just gave them an idea. What you're doing in any commission is a collaboration between your idea and your client's.
They can include illustration work like covers for e-books and print on demand books. Very often authors negotiate those commissions for themselves and not all covers are representational. Advertising or using signature links on author forums may give you visibility for those jobs.
Portraitists advertise and also display their art in local shows, whether or not it's for sale. Find out whether your local library and other public places such as coffeehouses may be willing to display your art -- sometimes they'll let you sell it off their walls too.
Advertising in local publications is a useful way to get word of mouth. So is socializing in local art groups and any groups related to your usual specialty -- if you do pet portraits, then giving out your card at dog and cat shows is a good idea. If you specialize in horses, advertise in publications the local horse fanciers read.
Whether you actively pursue commissions or not, the day usually comes when someone asks you for a price on a project they have in mind. If it's within your ability, quote a reasonable price a bit higher than you would ask if you just painted that on your own idea, unless it's an idea that catches your imagination so much you'd now paint it whether they bought it or not.
A good website and online marketing can do a lot to get commissions. Some thriving professional artists only market their art online. Figure out your costs of shipping and packing and charge those things at cost, or work them into the price. It's important to pack well when shipping any original art.
For small format art, I like using Priority Mail standard boxes and mailers, using low tack artists tape to put the art on an archival foam board backing, then tape a sheet of glassine over it and another foam board over it to make an art sandwich. I put that into an archival photo bag and then into the Priority Mail box and pack bubble wrap saved from supplies I ordered till the art sandwich is in the center and can't move.
After you have a commission
Be honest about the time it takes to do the project. Charge accordingly and be aware of the cost of materials. Work out how long it would take with a generous estimate allowing for things that can go wrong, life to interfere, various interruptions including holidays and random flu.
Then triple that estimate and give it to your client as the expected time for the commission to be done. I'm up front with my commission clients that I simply can't give a hard date for any completion, because my disabilities interfere so much with my ability to do anything that I have sometimes had whole chunks of a year unable to do anything useful. That "triple the time" estimate didn't come from disabled me.
It came as advice from a corsetier in New Orleans who did custom garments for weddings. It came again from an artist I knew on Jackson Square, one of the old successes with art in a dozen galleries and an "A" spot out on the park fence. Another professional artist recently blogged "multiply it by Pi" (a little OVER three times) the time needed.
This is so consistent that I think it's a general average for the amount of interference that anyone doing any project on a deadline will face. I have found it applying to projects I plan for myself even when I do take the season and my disabilities into account. It is the one method of time management that seems consistent across all types of work for hire. Triple the time estimate you worked out as generous and reasonable.
Occasionally you'll be able to do it a lot faster. When that happens, your client will be pleased and everyone will be happy. At worst, your client's still waiting for a paycheck or on a payment plan and you will have to store the finished art till the deadline, but that is not a big deal.
Ask for a substantial no-refund deposit to begin the work. That way if the client cancels out, you have at least something to show for your time -- the client's idea may not be generally marketable. Or it might, but you still can't know that it'll sell.
The other thing to work out before commissions is a policy to deal with changes. Sometimes clients will ask for changes that are as much or more work than the entire project was, not understanding the processes involved. They may think it's easy to make the baby fatter or put more detail into the face of someone in the middle distance. They may not realize changing a red vase to a blue one could mean starting over with the whole painting.
The best way to handle changes is to be upfront and honest about what work that change would entail. If it's minor, just do it on the first one. If it hasn't been done yet it's not always a big deal. If it's major and time consuming, explain that and have a schedule of charges for additional changes.
I wound up getting a commission once that paid one month's rent and helped me get through that month. By the time I gave up on it after the changes the client wanted, when she was getting nastier about them the closer it got to what she wanted, I had spent three months doing almost nothing but work on her project. It was hard to stand up to her and say "no," to the last demand. At the time I didn't think of charging for changes to the finished art.
I think back and if I had, the client might have stopped asking for more again and again. It wound up ruining one of the best oil paintings I ever did and ultimately left me burned out on oil painting.
So don't give yourself that heartbreak, set up a reasonable fee schedule for changes in advance and be sure your client understands that. It's very common for clients to assume that anything they want is easy because all they see is that your finished work is good. They don't know what all goes into creating it. Few understand just what is involved in changes, and often will ask for things that would mean starting over from scratch - leaving you doing two paintings for the price of one.
Learning to negotiate well and hold firm boundaries is important in self employment, but especially in commissions because you're dealing with people who think of you as magic. Capable of seeing what's in their minds and doing it exactly the way they would if they had your skills. It's too easy for them to imagine the impossible, not understanding that you can't scrape back to white if it wasn't planned from the beginning or that you can't add another two inches to the side of the picture if the paper's got only a half inch margin.
It helps to be able to explain what is involved in any change and to remember what your time is worth when quoting fees. Some artists charge more per successive changes just to keep it down to a dull roar.