Tomy,
Here is a link with an illustration that shows you how.
http://www.kidbow.com/how_to_use_a_bow_stringer.pdf
Unlike the illustration, however, I would place both feet an equal distance on each end of the bow's riser, before pulling up on the riser to bend the limbs for stringing, while bending the knees for greater leverage and to prevent straining your back.
"Shoulder width", like Chad (LBR) suggested, is about right.
Especially, with bows above a 50lb draw weight.
Lighter draw weight bows like 30 to 40 lbs can safely be strung stepping on the stringer cord with only one foot.
Be careful while sliding your bowstring loop onto the top limb tip notch.
You can twist a limb if you're not careful by letting the bowstring slide around the limb.
Definitely get a Selway stringer if you don't own one already.
Safest and easiest stringer to use by far.
Only one I will ever own!
Hope this helps.
Sag.