Some people use hot water and if that works for them great, but professional horners never use hot water and always use a dry heat, usually some kind of flame. The keratin in horn needs to get closer to something like 300F to become pliable enough to work and also loose it's memory of it's previous shape, if your heat is too low the horn won't lose this memory and will start to shift back over time. Heat gun as mentioned above is ideal.