Determine the work you want to have done.
Contact several masons. Ask friends and relatives for recommendations or check the yellow pages.
Make sure the masons you interview are experienced in the specific area of work that you want done. For example, if you want a fieldstone wall constructed, don't hire a mason whose primary experience is in installing foundations.
Have the masons come out to the site and quote a price for the work. If the work involves basic materials (concrete, block) it's probably easiest to have the price quoted for labor and materials. If the job involves high-end materials (bricks, fieldstone), you may want to have the price quoted for labor only - the cost of materials will vary considerably with your specific selection.
Ask for references from the contractors and contact these references. Were the customers happy with the work? Did the contractor complete the job in a professional and timely manner?
Pay particular attention to past projects when hiring a contractor to do expensive and craftsmanlike work (such as installing fieldstone or creating patterns involving cutting). Check out previous projects. Is the work up to your standards?
Make sure the contractors are properly insured.
Select a contractor based on price, area of expertise, references and your impressions.
Execute a contract specifying the work, cost, payment schedule, start date and anticipated completion date. Make sure the contract provides for the work to be guaranteed. Also include a provision that the contractor is responsible for cleanup.