Background wall help!

[HavenBby]

I am currently making some background but I need some help making a corner wall. I have no clue how to make it.

This is how far I have got, I need the corner to meet where the red mark is at.

Somebody please help.

[Cheyara_Skadi]

Do you mean like this?

[HavenBby]

Yes, please let me know how you did that.

[Cheyara_Skadi]

It depends on what software or programme you are using.

[Vernux]

What programme was used?