(1) File > New > New Sequences or use the Cmd+N shortcut. To ensure you have the correct Timebase (frame rate), Pixel Aspect Ratio, Field, etc., choose the Digital SLR 1080p30 preset.
(2) Click the Settings tab in the top left. Change Editing Mode to Custom and fill in the rest of the values as shown below. The key here is changing the frame size to 640x640.
(3) Save your sequence as a preset so you never do this again.
(4) Unless your footage is 640x640, you will probably get this pop-up window when you first drag footage on to your timeline. Make sure you select "Keep existing settings".
(5)
(2) Click the Settings tab in the top left. Change Editing Mode to Custom and fill in the rest of the values as shown below. The key here is changing the frame size to 640x640.
(3) Save your sequence as a preset so you never do this again.
(4) Unless your footage is 640x640, you will probably get this pop-up window when you first drag footage on to your timeline. Make sure you select "Keep existing settings".
(5)