How to make a favicon
Type one or two characters (a letter, your initials or an emoji), or choose a picture, then pick colors and a shape. The previews show how the icon looks at 16, 32, 64 and 96 pixels; the smallest one is what appears on a browser tab, so check that it still reads clearly. Press Download favicon.ico for the classic icon file, which holds the 16, 32 and 48 pixel versions, and download the PNG sizes you need. Then paste the lines shown at the bottom into the <head> of your pages.
Which sizes do I need?
A favicon.ico in the root of your site covers browsers that look for it by default. A 32 pixel PNG is sharper on modern screens, a 180 pixel PNG is used when someone adds your site to an iPhone home screen, and 192 and 512 pixel PNGs are used by Android and installable web apps. Simple shapes and one or two letters survive shrinking best; fine detail and thin lines turn to mush at 16 pixels.
Questions
Is my picture uploaded? No, the icons are drawn in your browser. What about emoji? They are drawn with your device’s emoji font, so they look like the ones on your screen; other devices show the PNG you made, not their own version. Does a picture get cropped? Yes, it is scaled to fill the square and the edges are trimmed; crop it first with the Image Cropper if you want control. Do I own the result? If you use your own picture or a letter, yes. Only use pictures you have the right to use.