Whenever possible the panels are tied to wall and to floor. If cleats fit without interfering with appliance that is easiest. Alternate are pocket holes and screws but you need something in the wall to screw into. You can cut a small hole and insert a piece of 1 by to screw into. Sub floor is adequate to screw into for the bottom of the panel.Attachments don't need to be earthquake proof but avoid toggle and the like.

If the panels are not flush sides then decide how you are going to deal with cabinets that but to it, was that 1/8 to 1/4" accounted for in the dimensions?, are there fillers between the cabinet and the panel to allow door to open if it swings toward panel?

IF the finished assembled unit will clear the ceiling when it is stood up it can be easier to build on the floor.It can also require a bit more thought and precision.

Install cleats for the bottom and back of the panels-PRECISELY. Good idea to leave a little room to shim at framework in the back of the cabinet instead of trying to get everything as a machine fit in 3 dimensions.

If you are going to scribe the panels for plumb and level THEN mark exact location on wall and floor , scribe then assemble. Many, if not most times panels are NOT ordered large enough to scribe though. That may mean some molding detail will be needed at the bottom and back.

Before standing the unit up, brace bottom front and diagonally at back prior to standing, make braces easy to remove later.

Pack back side of cabinet to panel when attaching.

You want two people to stand the unit up.

The same principals apply when building it in place. The challenge is supporting the cabinet while maintaining access to fix the panels in place.