Place your shelving unit against the wall. Mark the location of the mounting screw holes on the wall with a pencil. Remove the shelving unit.
Drill holes slightly smaller than the size of the hollow wall anchors, using a drill bit that's slightly smaller than the diameter of the hollow wall anchors.
Mount the hollow wall anchors in position on the holes. Tap them gently with a hammer to drive them into the wall. Remove the screws from the anchors.
Place the shelving unit against the wall and line up the mounting screws with the wall anchors. Place the anchor screws through the mounting screw holes and into the anchors. tighten the screws until they're tight.