Locate two stars you wish to use as subjects. Line the bottom of your cross-staff with the floor. You must hold the cross-staff straight to gain an accurate measurement.
Hold the cross-staff up to your face with the zero against your cheekbone. Close the eye on the same side, and focus on the measuring rod with your open eye. Move the slider until the topmost star is lined up with the right corner of your slider and the lower star is lined up with your left corner.
Remove your cross-staff from your face, keeping the slider stationary so you do not lose your measurement. Write down the reading from your measuring rod. Repeat the process to practice using your cross-staff, writing down your measurement each time to find out if it changes.