There are a few different techniques for forming a round loaf (also called a boule). This is the one that we learned when we first started making artisan loaves and that feels the most natural to us.
Dust the dough and your hands with flour. So we don't get too much or too little flour, we rub our floured hands gently over the surface of the dough to get an even coating on both.
Working quickly, use your thumbs and the palms of your hands to stretch the surface of the dough downwards. Meanwhile use while your fingers tuck underneath. (We find it easiest to use one hand to stretch and the fingers of the other hand to tuck, and then we switch hands to get an even surface tension all around.)
Flip it over and you should see a "belly button" on the underside. If this looks like it's unfolding, gently pinch the dough together to seal it.
Related: Good Question: How do I Store Bread?
(Images: Emma Christensen for the Kitchn)