In my humble opinion, there are two reasons that a manager asks for a self evaluation; they are sorrily incapable of performing an effective evaluation, or they are giving you an open ball field to position yourself within the company. It's been explained to me in the past that self evaluations help promote an individuals performance through ownership, but that comes out of purchased programs and has little to do with reality. I will not recommend the several formats that I had been exposed to, as I do not support any of them.
Regardless, I would put on your sales cap and remember that an employer has only two things to offer; monetary renumeration and education. Asses the balance of the two that will fullfill your needs and let the process begin. Structure it as if you where approaching a new venture, or better yet, as an independent consultant. Firstly, describe the current situation (in positive terms). Then let them know that you understand the core business mandates (typically described in mission statements). A short overview that demonstrates your understanding of the 'go forward plans', and finally, a short descriptive commentary on how you will contribute to the future development of the company. DO NOT look backwards and highlight shortfalls or list reasons for such. Think this, 'it's not about who or what you are, it's about what the needs of your client are'.
The above you will want to produce as an attractive, yet serious document. Now consider it's delivery. In such an important event, you will not want to drop this off in a mail box, or into a stack of paper on some one's desk. You will ask for an appointment to deliver it, by hand, in person. Plan on and request 1/2 hour, make sure you get this commitment. Tell them it is important to you if they want to reduce it's value. Remember again, the delivery is about their needs, but you should make it clear that you have needs as well, and that your objective is to fullfill the needs of both parties. You can decide if negotiations are appropriate, or if you are just going to give them notice that negotiations will soon follow. If that's the case, then indicated a time frame.
Depending on the management structure, you may want to cc: human resources and higher managing levels. Let your immediate manager know that this is happening.
Many employees and their employers do not acknowledge the importance of the evaluation. In effect, it sets the tone for the relationship. Make sure you play a heavy hand in setting that tone.