Hello there, I have a HD 1080p project and I would like to have a 1:1 format video at the end.
I'll have to crop some of my clips, so I would like to know

How to export to 1:1 format?
How to create a 1:1 mask so I can control what will be seen in the final image?

Thanks a lot, 

If you're using a recent version of Avid you can create a 1:1 project, import the bin with your sequence from your current project, and use FrameFlex to reposition your shots.

Thanks you for your answer, I'm a beginner with Media Composer, and have a recent version, but it seems that there is no "Frame Felx" tool??What can I use then?

How can I proceed? 
Can't create a 1:1 project, or I have to custom aspect ratio, wich modify the image...

Any idea? 

Thanks a lot,

Frame Flex is not a tool. You access it by right-clicking on the clip, choose Source Settings (top of the pop-up menu) and selecting the Frame Flex tab.