First of all, the building you want to build has been completed.
Move the picture to the location where you want to build it.
(Mouse) Left-click the box where the building is ready,
Click the building mode button on it again, don't put it down,
Drag it out, drag it to the ground,
Right-click and press two keys at the same time.
That tall building! It is flat!
If this building is a barracks,
You can build a spy directly by building it directly on the enemy!