Use a plain, light-coloured background with clear contrast to your face.
Include your head and upper body and do not crop it: the service crops the photo for you.
The photo must have been taken in the last month, without glasses unless you have to wear them.
How to do it with Requisito Listo
Open the «UK passport photo (online)» template in the app.
Take the photo with the on-screen guide (or pick one from your gallery).
The app centers your face, makes the background white and gets the exact size and file weight.
Check the checklist, then upload or save it.
At the end you'll see a checklist: size, file weight, background and format. If something doesn't comply, it tells you what to fix before the portal rejects it.