Step 7: Never Trust a Factory Edge Without Sqaring
When you buy sheet metal it will have crisp, straight (on some axis) lines. Before you start laying out a project, take the time to check the edges for square. I use an aluminum T-Square for this. A carpenter's square, machinst's square, etc. all work as well. This will save you from tearing out your hair after 4 hours worth of work when your project is 1/8" off center!
Another option if you're buying your sheet metal from a specialized sheet metal supplier, they can cut the sheet on a squaring shear, a huge hydraulic monster machine that makes perfect cuts.