How to fix an out-of-square rough opening before installing a pre-hung door in an old house

When renovating an old house and installing a pre-hung door, the biggest fear is an out-of-square rough opening. The door slab itself is fine, but after installation, the gaps are uneven, and the client will think the installation is sloppy. Before installation, I first measure the width, height, and both diagonals of the rough opening, and then use a level to check if the studs on both sides…