There are several reasons an employee may need proof she works for you. Usually, it's because someone -- a landlord, for example -- wants confirmation that your employee has a job and a steady income. Foreign employees may need a verification letter to get a visa. Writing the letter isn't complicated, as long as you're careful not give away any confidential information.
Templates and Forms
Check whether your human resources department has a standardized template for verification letters. It's a common practice, particularly at larger firms. The template makes it quicker for the writer to finish, and HR doesn't have to worry about the letter saying something wrong. If the letter is to qualify the employee for some type of government benefits, the agency involved may have a standardized form. It's up to the employee to bring the form in, after which you fill it out.
Keep It Formal
A verification letter isn't friendly, and it isn't creative writing. It's pure business, so send it out on your company letterhead, written in standard business format. Sign it like any other business letter. A subject line -- "Employment Verification for Jane Doe" -- lets the recipient know the topic. The text is where you tell the recipient that Jane Doe does indeed work for you, and what her salary is, if necessary. When writing to immigration, you may also need to include details about how long she's worked for you and what her job duties are.
Follow the Rules
Check with your HR department not only about available templates but any procedures it requires you to follow. It may be a company rule, for example, that before you send someone employee information, the employee has to authorize it in writing. Confine the letter to the bare facts you're asked for: If the letter doesn't ask for salary information, don't provide any. Avoid off-hand comments about an employee's personality or performance that could come back to bite you.
Things to Watch For
Verification letters sound routine, but important decisions -- does your employee get an apartment or an insurance policy? -- may hinge on them. Don't just throw out answers: Get the facts about employment dates or salary from your company records before putting them on paper. (Proofread the letter before you send it out, or have HR do so. Mistyping $50,000 when your employee's salary is $60,000 could affect whether she gets the apartment.