First, make sure you screen is square. That is a prerequisite to me.
Then you need to use a spirit level on the projector to make sure that it is perfectly level, front to back and side to side.
Then you need to zoom out until the entire focus pattern is a little inside the screen border and then use physical twisting on the mount until the left and right sides are parallel and square to the screen edge. All the while making sure the projector remains plumb.
Then once that is done you would use the zoom and shift controls to align the screen to its final place.
It can be time consuming but it is pretty simple and you will deffo benefit from another body near the screen to call out how it is looking as you go.