Slide From Invisible Wall

How do I make a slide start from any point I want? For example, a slide onto the screen appears out of the edge of the video, how can I make it appear from let's say 1/3 of the screen (left-right)?

Cropping on the Inspector, seen on the Edit Page. You could also do it by creating a Power Window and using an alpha to cut a matte that reveals the image. And you could also do it in Fusion, but there's a bit of a learning curve there.

This is all covered in the manual and in the basic texts on editing available for free on Blackmagic's Training page:

https://www.blackmagicdesign.com/produc ... e/training