Either contact the Better Business Bureau, a friend, or neighbor, and or speak to a local building contractor and have them recommend at least three reputable licensed subcontractors for each trade.
Provide each subcontractor with a full set of working drawings and have them provide you with an estimate. Give them all a deadline to which they must reply by. Have them come out to the site and take a look at the existing conditions in which they will have to work under.
Have each subcontractor provide you with a detailed written estimate of their proposed work (so you can compare apples to apples). The estimate should include all permits, all insurance (Workman's Compensation and General Liability), amount of time required to complete the work, and final clean-up. Compare each estimate for inclusions and exclusions.
Have each subcontractor supply you with at least three verifiable references. You must do your due diligence and call these references.
Sign a contract with the best qualified subcontractor for each trade. A reputable company will not request money up-front. The payments should be in installments, upon progress of work or in one lump payment after the work is completed.
Work is to begin only after their respective permits are acquired. Final payment should be made only after a final building inspection has been approved by local authorities. In order to induce final payment from the owner, each subcontractor needs to provide the owner with an unconditional final release of lien from his company, his vendors, and suppliers, thereby assuring that all labor and materials have been paid for on this project.
If you, as the homeowner, are going to act as the general contractor and subcontract all the different trades, you must schedule and coordinate all activities in order to ensure a steady flow of work and to prevent any conflict between the trades. This is usually achieved by a predetermined written schedule and regular coordination meetings with all the different trades being present.