You start with your own details: name, home address, job title and a personal phone number or email you will check while off work. The next screen asks for the employer's name and address and the person you are writing to, usually your line manager. The preview on the right fills in as you type.
The absence screen does most of the legal work. You give the first day you could not work, say whether you know your return date (the date field disappears if you choose "No, not yet") and indicate whether the certificate is enclosed or will follow. The help text explains the carry-over rule (section 5(4A)): statutory sick leave does not apply if, on your last normal working day before the absence, you were already on statutory sick leave from the previous calendar year or on sick leave covered by Illness or Injury Benefit.
Two short questions then decide the claim paragraph: whether you have 13 weeks' continuous service, answered as at the first day of absence, and whether your contract or workplace provides its own sick pay. The last questions cover the delivery method and the date, followed by a review screen. Use Next and Back to move between screens, then Finish to generate the letter, which you sign and send, with the certificate if you have it.