Choose a photo, move the circle onto your face and save it: as a square JPEG for apps that cut the circle themselves, or as a round PNG with see-through corners.
Square or round file?
Most apps want a square file and cut the circle themselves: save the square. A round PNG with see-through corners is for places where you place the picture yourself, like a slide, a document or an e-mail signature.
Fit your face in the circle
Your face a little above the middle, filling about half the circle.
Room above your hair, the top of your shoulders in.
Check the smallest preview: that is how most people will see it.
Questions
Is my photo uploaded?
No. The crop is made on your own device, in your browser.