It is a good idea to provide a "Letter of Employment" to all new employees. This can also serve as your offer of employment. It ideally should be provided before the start of employment and should set out the duties and responsibilities of the job, wages or salary, hours of work, leave entitlements and other relevant items. Include a copy of the job description with the letter.
If your letter contains any special conditions or accommodations, it would be helpful to have an employment lawyer take a look at the offer before you make it.
When the position has been accepted in writing, send out polite rejection letters to the other applicants.