Keep it short and just pick out the key points you really want to emphasise, directing the recipient to your c.v. for full details.
In particular, read through the advertisement for the job and pick up on the key points. For example, if the job specification calls for experience with Microsoft Word and Excel, together with supervisory experience, make sure that you mention each of those things.
e.g.
"My previous employment has developed my skills with Microsoft Office programs, in particular with Word and Excel.
While I am a good team-player, I also enjoy taking responsibility and accepting new challenges. My supervisory experience with Bloggs and Co has enabled me to develop my natural skills in this area"
Chris