There has been several threads about how to flatten a cow horn on this website, so a SEARCH can also help you.
I have made several hundred flattened powder horns and other flat horn items and I have found that hot cooking oil at a temperature of about 325 degree's works ideal for heating and flattening horns.The key here is to keep dipping the horn in and out of the oil and let it slowly heat-up until pliable without scorching the horn. Some horn makers like to use pure lard and others, like myself use cheap cooking oil. I've used both and see no real difference. It's just your choice. For a heating source, I use a "Fry Daddy" deep fryer. These can be bought at any Walmart for around $20 or yard or garage sales. Basically speaking, not any horn will make a good flat horn, as you want a horn with even thickness walls and not overly thick. Wall thickness of less than 1/4 inch seems ideal. You do have to make a wood interior form in the shape of the horn interior to push into the hot horn while flattening takes place to keep the horn walls from collapsing inwards. I flatten most of my horns in a heavy shop vise between boards. After you heat the horn as above, you quickly place the horn in the vise between the two piece's of wood and slowly start cranking the vise closed. Go slow and don't over-force. You may have to re-heat the horn several times to get it to the flatness you want. Let completely cool before washing with soapy water.
I went through this procedure quickly, but I think that you might get the picture. Many of the good folks on this website own and use my flat horns and should be happy to show you the styles I make.
Rick