Just go out and dig a hole in the ground, pad it with sand (old carpet works very well too) to even out the rough spots and protect the liner, and then install the liner. When you've filled the liner with water, add a few plants from a local nursery and you're done. Oh yes, one more important thing, you'll have to spend lots of time watching all the critters.
If you want something a little more complex, that's quite easy too. Make the difficult job of digging the hole easier by spreading it out over a few weeks. If your soil is clayey like mine, spraying the excavation edges with water after each digging session will loosen it up for next time. If you're using a preformed liner, place it on the ground first to mark the shape you'll dig. With a PVC or rubber fabric liner, be sure not to dig the hole so large that there isn't enough liner to cover the hole and some distance back from the edge.
If you're like me, you want to attract frogs. The best way, the experts say, is to leave fish out of your pond. They'll eat most of the frog eggs and tadpoles. But, I like fish! Besides, you can have fish and some frogs, too, by creating lots of hiding and feeding cover for the frogs. I put some leaves and branches in my ponds. These provide cover as well as added nutrients and structure for small tadpole and frog foods to prosper.
Piles of rocks, emerging as islands, and healthy emergent vegetation are also important as they provide emerging and perching places for frogs and for other pond life like dragonflies. Don't make the sides too steep as it will be very difficult for the frogs and other animals that may venture into the pond to escape. With many nooks and crannies, like in a natural pond, the frogs will have a chance.
You can jumpstart your pond with frogs, if you'll pardon the pun. Contact a local nature center or resource agency and find out about tadpole relocation efforts that may already be underway. Ask if you can obtain some tadpoles for your pond. If you don't want to go to all that trouble, worry not as chances are the frogs will find their own way to your pond. As you can see, making a frog pond is simple. Yes, it takes some effort and time, but I found just creating it to be almost as much fun as enjoying it afterwards.