Clearly describe the work needed in writing, or draw sketches to provide to the plumber. Be sure you understand exactly what you need. Take advice, but don’t be afraid to ask questions and have them explain their reasoning. An experienced plumber will be happy to do this.
Always ask for a quotation, not an estimate. An estimate is just a rough guess of how much the work will cost, and it is not binding. They may need to provide an estimate if full information is not available at the pricing stage, such as if the work cannot be properly surveyed, but an experienced plumber should be able to give you a fixed price for the work.
In all but emergency work you should expect to receive a fixed price in writing prior to the work taking place. The quotation should provide detail on the range of works to be carried out and the price for that work.