Warm up your voice. Begin with a comfortable scale and sing upwards, note by note, until you reach the top of your comfortable range. Repeat this exercise singing downwards. Continue until your voice is steady and clear.
Play, or have your accompanist play, middle C. Play upwards, singing every note until you reach the end of your comfortable range. Mark down the last comfortable note. This is the upper boundary of your vocal range.
Play middle C again, then progress downwards, singing every note until you are no longer comfortable. Mark the last note of your comfortable range. This is the lower boundary of your vocal range.