It takes time and practice but this is how I was taught with a whetstone. I usually use a bit of oil on the stone I use. I hold the blade flat and angle it until the edge and stone meet and I can see no air in bewteen. Then I draw the knife like I'm trying to shave a tiny sliver of the stone off. I switch sides each time and then check the edge after about five strokes on each side. Then I switch to a smoother stone if I need to and do it again. The oil helps because its something I can see being "shaved" off the stone.

Once you get a feel for it you are good to go!