Most employers do not ask for writing samples at the beginning of the recruitment process when you send out your initial resume and cover letter. Do not submit one at this stage unless asked. More than likely, a writing sample will be requested later in the process, so you should have one prepared.
Your writing sample should be the best legal writing you have done. As a general rule, 5-10 pages will be of sufficient length. It can be a memo from a summer job, the writing competition note you submitted for the journals, a portion of a moot court brief, or part of a memorandum or brief that you wrote for Lawyering. If at the time you are applying you have a law journal note or a seminar paper, use that. Only use work from Lawyering if you did well on the assignment, and you feel that this first year effort reflects your current ability.
Additionally, you should proofread the document, check your bluebook citations, and make the changes recommended by your Lawyering professor. Once you have made the suggested changes, your Lawyering professor may review work from the Lawyering class. If you are sending something you worked on for an employer, be sure to obtain (and make clear to the prospective employer that you have obtained) the employer's permission to use the materials. Be very aware of confidentiality issues with memos and exclude client-identifying information. If you are working on a journal note, you might send a discrete 10-15 page section, with a synopsis of the balance.
Your writing sample should include a cover page. Write your name, contact information and law school name on the cover page. Also state the circumstances under which you drafted the document. If you are sending a sample that has been edited by someone else, indicate the circumstances. (Be aware that some employers, including judges, request a sample that has not been substantially edited by another person.)
You should also be sure to make clear why and when you wrote the sample - e.g., for a seminar in a particular course, as part of a memorandum for an employer, for a particular journal. (If you redraft an earlier effort, you should describe the sample as “based on a memo I wrote in our first year writing program”.) If your writing sample has been accepted for publication be sure to indicate that. If you are using as your writing sample an opinion that you worked on for a judge (for example, in a summer intern position), do not use the phrase “opinion that I drafted” or “opinion that I wrote”. Instead, indicate that you “worked on” the opinion. Be aware that some employers may not accept an opinion, or any other writing ultimately attributed to someone else, as a writing sample. Speak with someone in the Career Services Office if you encounter any difficulty in selecting a writing sample.