What to have ready
The passport you will travel on, a photo of your face (you can take it with your phone), an email address and a payment card.
The details must match the passport exactly. A name spelled differently is the most common reason for refusal.
The steps
Go to the official GOV.UK site or download the official app, scan the photo page of your passport, take the picture, answer the suitability questions and pay £20.
Then you wait for the decision. It usually arrives within 1 day, but allow up to 3 working days.
Check before you submit
That the passport number and expiry date are copied correctly, that your face is clearly visible in the photo, and that the email address is right — that is where the decision goes.
Apply well ahead of time, not on the day of departure.