You are more than welcome to be as involved with the service as you wish. If a service is taking place in Church, then the local minister in charge of the will be present, but allow you to deliver a eulogy or read a poem. If the service is at the crematorium, which is a public building, you can deliver the whole service yourself if you wish. We would, however, recommend that someone acts as a Master of Ceremony, just to offer guidance, and take some of the pressure off you when having to play music etc.