What you need
A phone, a plain light wall, a window with daylight and a helper (or a tripod). You do not need a studio. Most countries only reject photos because of shadows, bad cropping or a wrong background, and all three are easy to avoid.
Set up the shot
Stand about one and a half metres from a plain white or light wall, facing a window so light falls evenly on your face. Keep the phone at eye level, not tilted. Turn off flash, because it creates glare and red eyes.
Pose and expression
Look straight at the camera with a neutral expression and your mouth closed. Keep your shoulders square, hair off your face and glasses off if the rules ask for it. Take several shots and pick the sharpest.
Crop and size it
Use the maker for your country: US, UK, Canada, Australia, or see all countries. Zoom and move the sliders until your face is centred, then download the JPG.
Check before you submit
Plain background, no shadows, eyes open, head the right size for your country, and a file small enough for the form. If the portal has a size limit, use the 200KB tool. Rules change, so confirm them on the official website.