This article is from the Recreational Figure Skating FAQ, by Karen Bryden with numerous contributions by others.
While gliding forward on one skate, bring the free skate in toward the
heel and turn the skate perpendicular to the one on the ice all in one
smooth motion without lifting the free skate more than an inch or so
from the ice. Firstly, remember that it is the outside edge of the
braking skate that touches the ice. The braking skate should be placed
so that the middle of the blade intersects with the skating foot's
blade, that is you are forming a perfect T. You want your foot far
enough back so that you do not step on the blade of the skating foot
(disastrous results will ensue if you do), but close enough so that
you can comfortably gradually shift your weight onto that braking
skate's outside edge.
Don't forget to practice with the other foot as well. Do them going
quite slowly until you get a feel for the balance, then pick up speed