drill a hole into the end of the horns.put a dowel into the hole. then make you a skull plate from a board,drill it at the distance you think they were on the skull and at the angle. push the dowel into the board.leave the base of the horns about 2" from the board. get some ( "rockhard puddy ") it comes in powder form,get at local paint or hardware store.mix it with water to about like mashed patatoes,then put it around the dowel up to the horn base.shape it to look like one of those antler plaques.wet your fingers as the putty hardens,and smothe it the way you want.

you may just take a board and two dowels sticking out and pratice on that first.draw an oval shape on the board to go by.pratice first.that will help with the finished product.after finished,cut the board around the putty so the felt will fit.

I have made a number of these and when finished,I would get some felt and put over the putty(looks real good).you can use contact cement to make the felt stay in place.cut it to fit befor putting the glue on it.after finished,you can put on a plaque like you want.
When finished , most people want be able to tell it from the real thing.

hope this helps,if not just ask,I will try to help.