Sorry , my English is poor so I will try to explain my problem so easy as I can.
- I create and modify Plane.

- Expand it up.

- And now I would like to get this 45deg slant.

- to expand verticles right to get frame corner.

So far i have been doing it like this:
It works but it is tedious. It can be done manually for a dozen or so vertices, but not for several hundred. I try boolean modifier but I am not satisfied with the effect. I'm sure there's an easier way to do this type of modification, what is it?




