As with any DIY project, start with gathering all yours supplies into a designated area. (i forgot the scissors here!)
I like to use an old picture as a glass workspace for my clay projects :)
Make a horn shape by rolling a long strip of tin foil into itself. bend it at angles to get a curvier piece. Experiment!
keep squishing the tinfoil into a more rounded, horn like shape, add more foil if you want more bulk on the bottom, which i feel is better for balance. Repeat steps one and two to create the second horn base.
This way any shape can be made, small or big, curly or straight..whatever you like/nee for your costume.
Take your polymer clay(enough for just the first horn) and knead it for 2-5 minutes, however long it takes you to get it nice and soft.
Either with the rolling pin to roll it flat or with your hands to knead and push it flat, roll the clay out so it it's about this large.
Begin wrapping the horn, cutting the clay where needed to get a complete cover.
Smooth the clay to a nice, rounded and fairly even shape around the clay.
Don't worry about covering the very bottom if you plan to mount them on something.
Here's a picture of the two horns at this step, you can see the clay makes it bulkier.
After completing the second horn to this stage, lean the horns over a rounded object and make any necessary shape modifications to help them match.
Now let's let them sit a few days or pop them in the fridge for a few hours to stiffen the clay back up.
Using a sharp tipped object IE modeling tools, sharpened pencil, ballpoint pen, etc, score circular indentations around the body of the horn an you would see in an animals horns. (this is optional if you want a much smoother looking horn)
Here's an example for those who don't see a lot of animals.
Bake as directed on the clay package's instructions.
At this point you have two mostly matching clay horns. To get a more distressed look, sponge black acrylic paint over them to get the bone texture you see below (i used tan as a base there.)
If you have not yet figured out what to do with them here are a few options.
To attach them to a pair of headphones;
Measure three pieces of ribbon.
one short one that sets the length that they will sit apart, and two that are the same length that you will glue flat under the horns as to tie them on to the headphones.
Attach the horns to the edges of the spacing ribbon with hot glue, then attach the trying ribbons at the middle of their length flat under either horn.
It's a little awkward to tie them on the first time but it's well worth it.
To make them into head decorations,
hot glue the horns at the desired distance apart to either a headband or a very long strip of ribbon or cloth as to tie them on to your head.