- Place a belt and drag your mouse in the direction the belt is going.
- Drag it in the opposite direction.
- press R to rotate the belt by 90°
- Drag perpendicularly
Magically belt between mouse position and "end of belt" are placed.
A video might explain better what I'm saying:
The expected output is rotation of the belt that was hovered by the mouse and placement of a new belt perpendicularly: