Measure the shelving and mark the cut line with a marker
Remove the shelf from the brackets or supports
Wear safety glasses and gloves
Clamp the shelf securely to a workbench
Use a hacksaw, bolt cutters, or an angle grinder with a metal-cutting wheel
Cut slowly and steadily along the marked line
File or sand the cut end to remove sharp edges
Check the fit and trim again if needed
Reinstall the shelf after cutting