sampling and adding other instruments

Hey wats up fp i got a question that i need help with. so here goes, my question is when your using a sample from song or sample pack and you know the key of it how do add other instruments to it with out it sounding muddle? i want it to sound cohesive? so if i have horn sample and i want to add piano to it do i work with the volume of each or make each sound be in the same pitch with each other?

any help would be great thank u