VaderHaider,

Technique neesd two track in timeline:

Upper (lower number) is the background which will be visible through the transparent areas.

Lower (higher number) has the foreground clip which was filmed against a plain blue or green background.Select this clip and use 'Effects' >> 'Video effects' >> Chroma key' and then blue box or green box as needed.

Quality of results depends a lot on how even the green or blue is. Details may vary with program version. It would help if you include this information with your question.

Regards,

Carl