Measure the distance from your electrical box's knockout hole to the corner or the location which will be the center of the 90-degree bend. Transfer this measurement to a piece of conduit and mark it with a permanent marker.
Insert the conduit into the hole in the hand bender's curved head. Hold the end of the conduit nearest you in place with your foot.
Pull back on the conduit's handle. Continue to pull until the conduit lines up with the 90-degree mark on the hand bender's curved head. Stop bending and slide the conduit out of the hand bender.
Lay a level on one of the conduit's ends. Check if the conduit is level. Move the level to the conduit's other end and check that that end is level. Insert the conduit back in the hand bender and adjust the bend as necessary. Once both ends are level, you have a perfect 90-degree bend.