There are obviously many variations of frog flies out there, given their popularity for bass and other warmwater species. This particular one is inspired by something I saw from another angler, but with modifications to the look, shape, and materials.
Here's a quick breakdown the materials to start off:
Hook: Size 12 bass bug style
Thread: 6/0 olive color
Tail: Chartreuse hackle feathers, barred olive hackle feathers, olive marabou
Body: White craft foam, green craft foam
Extras: Super glue, chartreuse rubber legs, stick on eyes, green Sharpie pen
When it comes to foam, really any durable product will do. I personally like to buy my foam from general craft stores, as opposed to fly fishing shops, as the cost markup for fly fishing specific materials is huge. At a craft store I can get a standard computer paper size sheet of foam for about $1, while a 3" by 3" square of the same stuff at a fishing shop will probably run you about $3.
So let's just into the tying part, shall we?
Step 1:
Set down a nice olive thread base on your hook
Step 2:
First tie on a small clump of olive marabou, sticking about 1 inch past the hook bend. Now select 2 chartreuse hackle feathers, and 2 olive hackle feathers, each about 2 inches in length, and fairly slim. Make 2 feather pairs, each consisting of 1 feather of each color. Make sure one pair is aligned to splay out left, and the other right. Carefully line up the feather lengths and combine all 4 to be held pinched between two fingers. Press the tying side of the feathers onto the hook, in front of where you tied the marabou on, and then carefully wrap them on. It's usually best to do one or two loose wraps first before tying more tightly, as this will ensure that the feather stems don't twist right away, ruining your alignment.
Step 3:
Now we need to create the foam top and bottom that will combine to be the body of the frog. What you are aiming for is a roughly pear shaped patch of each, seen in the picture below. For this size 12 hook, each cutout was about 1 inch long, or perhaps a little longer. If you wanted to make a larger frog fly, you would maintain the same rough foam shape, but just make it longer and wider. Using the white foam patch, tie the slim end to the bottom of the hook shank so that the other end of the foam is pointing towards the hook bend. The slim end is really just for fastening the foam on tight, so you will want to wrap most of that end on. Now do the same thing with the green patch, but this time on the top of the hook shank. Take a look at the second picture below to get a sense of what you should have by the end of this step.
Step 4:
Select a medium thickness olive saddle hackle. Move your thread placement directly behind the foam now, which you can easily accomplish by pulling a tight wrap through the spot where the white foam meats the green, on the side of the hook shank. Do a couple tight wraps there to secure its new placement, and then tie in the back end of the saddle hackle. Now do about 3 or 4 turns of the hackle around the shank to create a little more body, and then tie it off. This is the last of the thread wrapping, so you can now fully whip finish.
Step 5:
Take two strands of chartreuse rubber legs, pair them together (lengths aligned), place the mid point on the top of the hook shank, and pull the ends tight towards the back of the hook so that they rest securely at the back of the foam and pointing towards the bend. Now take your super glue, and paint a line with it around the entire inner edge of the white foam. Before the glue hardens, pull the two foam pieces together at the hook shank like a sandwich. Hold them firmly together for about 15 seconds while the glue hardens. Tough up any gaps between the foam with more glue if necessary, and also place a drop or two where each pair of rubber legs comes out, so that they are also secure.
Step 6:
Take two sticky eyes, and secure them to the top of the green foam with a dab of super glue.
Step 7:
With a green Sharpie pen, mark up the top and bottom of the foam to give the fly a more natural effect.
AND THAT'S IT!
Our fly is complete and ready to be fished.
Foam is obviously very buoyant and doesn't easily get waterlogged and lose effectiveness like spinning hair, so you can use this without sinking issues for hours on the water.
As with any top water fly, the best way to fish this is with short strips and popping manoeuvres. Cast it by lily-pads, the shore, or other stops on the water that frogs would likely be seen. If you find yourself commonly hooking onto too much vegetation, you could also tie this with a weedless hook guard using wire or thick tippet line (30lb or something similar).