How do gel analysis with Imagej?

I am a student, I need to use Imagej for quantify my protein on the gel.
i see the tutorial, in this link, but I have some problems
I select my first band as the first and then copy the same for the other band and I assign them the progressive number with Ctrl+2; but when I want to create my plot (Ctrl+3) there is always another rectangle. How can I resolve my problem?
Any suggestions or idea? Which video I can watch? the image must have a specific format? .jpg okay?

Thanks for your help

Dear @student,

when you have moved your selection to the last lane that you want to analyze, you don’t have to do Ctrl + 2 but Ctrl + 3 to show the result. Another possible explanation is, that after creating a selection and pressing Ctrl + 1 a new rectangular selection is created automatically.

A quick search also revealed the following tutorial video on the topic: