Sequence of step for Chroma Key to work consistently

[Dotted 8th note]

So I've successfully done about 3 videos with a green screen. Each time, it's a bit of an experimentation, trying to understand the steps for it work versus what I've set and what I've clicked on...

1. Import green screen video

2. Import background pic or video

3. Click Chroma key icon, drop onto green screen video. 

4. Click eye dropper , click on green area of background.

5. Use sliders to adjust.

This SEEMS like it should work, but once again, I'm banging my head , trying to understand why I get either nothing but the background pic/track or background video/track to play, or, the foreground green-screen video to play. There is some kind of sequence thing or something which is infuriatingly not mentioned or covered in any tutorials that I'm apparently not doing correctly. Thanks for any help.

[Dotted 8th note]

omg,...for whatever crazy reason , this workd..... I tried grabbing the background track, and dragging it up ABOVE the background pic track and low and behold , it works! So greenscreen track is on 4, background track is on 3, where before the greenscreen track was on 2.....The question is WHY? Why does the greenscreen track have to be ABOVE the background pic/video track for both to display?

[Steve Grisetti]

Here's my tutorial on working with green screen, Chroma Key and Videomerge.