Alright guys. Where doing it. Where making this hapen.
As I have no need or desire to make another set of Nitram horns, I'll just be showing you how I made my horns on a smaller scale (with some photos I took while working on Tavros interspersed), but the techniques can be applied to any of the trolls (and is very similar to how I made Ahab's Crosshairs). I've also just made some of the smaller horns out of Sculpey using a lot of the same methods since I personally find it easier to sculpt small ones instead of carve them, but foam versions will obviously be much lighter.
First thing's first. Before you begin, draw the horns out on a piece of paper and hold them up to your head to make sure you're working at the size you want. It's waaaaay better to go through a bunch of paper making sure they're the right size than to notice halfway through making them that they're too big or too small and you have to start over or buy more materials. Not to mention that once you have a paper template at just the right size, you can use it to match your horns to so they come out the same size.
Pink polystyrene insulation foam! This can be found at Lowe's or Home Depot or most hardware stores. It usually comes in 8 foot sheets of various thicknesses (and costs). With the big sheets, I always ask an employee near the lumber to cut it in half for me so I can fit it in my car. Though recently I've found they sometimes have these smaller squares pictured above! They aren't covered in plastic film (which means you don't have to waste time peeling it off) and they're a little cheaper too!
I traced my paper template for Tavros onto the sheet like playing Tetris so I could waste as little as possible. They took several sheets.
A lot of adhesives and nearly all spray paints will melt pink insulation foam so you have to be careful about what you put on it before it's sealed. Before you try anything different, you should probably google it to make sure it's okay to use. I use Super 77 spray adhesive to glue my layers together. Just make sure you spray it from a good distance and more let the adhesive fall onto the foam rather than spray close directly at it. Let it sit for about 10-20 seconds before sandwiching the foam together, it'll stick better. Super 77 will adhere to most things so try not to get it on anything else!
As you can see from the scrap above, Tavros's horns were 4 layers thick.
To cut through the multiple layers of foam after they're glued together, I use a cheap turkey carving knife (just make sure you don't use it for food after this!). I'm just gonna make a horn out of that scrap from Tavros. If you drew your horns onto the foam, be sure to leave extra space around the lines when you cut it out so you have room to carve down from otherwise the horns might end up a lot smaller than you intended by the time you're done sanding.
Carve out the shape using your xacto/boxcutter of choice and then sand it smooth. If you don't want to add texture to it, skip ahead to the sealing and painting steps!
Also be prepared to be literally coated in pink dust and slivers of foam during this stage. You will need to shake out your clothes and wash them separately to get all the foam off. And you will still find pink bits on the floor weeks later. Stuff has static cling of doom.
To add the texture, fold a piece of coarse sandpaper in half and just drag the folded edge across the horn. Make these cuts irregular. Some can be short, some can be long to wrap all the way around the horn, some can be perfectly horizontal, some can be at slight angles.
And then KEEP DOING IT.
After a while, the cuts will build up and start to look more organic. You may want to purposely go back over a few of the cuts to make them deeper and provide more variation.
Continue until you're satisfied! Then it's time to seal the foam for painting.
You'll want to coat all of the foam in 1-2 coats of Mod Podge. Be sure to get it into all the grooves. (If you're making a prop that you'll be spray painting, do several more coats so the spray paint doesn't eat the foam.) If you are making large horns, A HAIRDRYER WILL BE YOUR BEST FRIEND.
If your horns are textured, lay down an undercoat of brown on the bottom, red in the middle, and orange on top. Then you will lightly drybrush the raised areas. Red over the brown, orange over the red, and yellow over the orange. This way the darker colors will remain in the grooves and give the paint depth. Obviously if your horns are not textured, you can just go with the red/orange/yellow gradient or solid bars if you prefer.
After everything's dry, I give it a coat of clear matte spray paint. Then clean up the bottom of the horns and round them out to attach to a headband. Or you can run a shoelace through the base of the horn on smaller ones and lace it through your wig cap.
Happy horn making!