As well as a cover letter, you will likely need to submit a resume or CV (curriculum vitae).
You should describe your relevant work experience in reverse date order. Include a brief description of your duties and responsibilities for each job and, outline your key achievements for each role if possible
Include your education and training, and perhaps an outline of relevant out-of-work activities.
Include two referees (who may be contacted at any stage of the recruitment process), along with their work address, e-mail and contact telephone number for this purpose.
If you can include your current line manager as a referee, brilliant!
There may be verbal discussion with your referees or they may be provided a referee report template.