You don't, but it won't work until the answer is three fists or less, and glancing at the sun in the evening isn't going to hurt you for the time it takes to put your fist/fingers next to it.

cover the sun with your left hand and position it so the bottom of that hand is just covering the bottom of the sun. then start measuring with your other hand, from the sun to the horizon

I would think that a single quick glance in the general direction of the sun won't do much harm