This post will tell you how to make the horn for your unicorn
Roll a blob of epoxy putty
Now make the horn. Make it by rolling the blob in your hands, roll it more on one side than the other to create a conical shape. Now twist it round in circle to create the twisty shape of the horn:
Now add a small amount of putty to the forehead of the horse. Flatten it down.
Use water or smoothener to smooth it down so that it is nice and smooth against the horse
Now place the horn onto the centre of the horse and smooth down the edges with the milliput you have added using a sculpting tool
Now leave your model to dry