Once you spot the open siphon, place the plunger directly on the hole, and vigorusly work the plunger up and down. When you push down, the water in the plunger blows sand/mud out. You'll end up with a hole maybe a foot across and straight down for about 18 inches or so for steamers, about 6-10 inches down for haugs. When there is a slight current, always stand on the "upstream" side of the hole you're plunging and the water will carry away all the mud and silt you'll be kicking up. A cheap pair of polarized sunglasses is a good item to have too.
The clam won't always pop up out of the hole, so you'll have to reach down into the hole and feel around. Beware the broken shells of old clams. They'll do a number on your fingertips, but the salt water heals the cuts up pretty quit. Use this method, and in a couple of weeks, you'll be able to tell what kind of clam it is by the shape/size of the siphon. Another nice thing about this method it that it's VERY relaxing.
I'm usually good for about 1/4 bushel each time. Not a big amount, but considering the relative small amout of effort required, it's a great way to git them clams!
PS. A windy day can wreck the whole process because it makes it very difficult to see the bottom.