This area will be expanded, but for now here are a few important things to help you take good green screen photos.

Do not us auto white balance.
If your camera's white balance can be set, make sure that it is set to flash or sunlight, not to auto. The auto setting is confused by all the green in the photos.

Keep shadows off the green screen.

Shadows will show up as dark or black areas on the output photo. To avoid shadows make sure the green screen is tight on its stand and that there are no folds.

If you use a camera flash, make sure that the subject stands away from the background. You do not want a hard shadow cast on the green screen.  If this is a problem then take the photo outside with no flash, or in a room with a lot of light.

It generally works better if you do not show feet.

Take you photos from the knees up. If you must show feet, have them stand on a small rug. The reason is that the shadows that feet would put in the photo are hard to get right.