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The first step in joining a threaded carbon steel water pipe to a fitting involves applying pipe-joint compound or tape. This is crucial because the compound or tape helps to create a watertight seal by filling the small gaps and imperfections on the threads, thus preventing leaks. The application of pipe-joint compound or tape can also help to lubricate the threads for easier assembly, ensuring that the connection is both strong and secure.

In this context, options such as installing a gasket, soldering, or welding would not be appropriate initial steps for joining threaded carbon steel pipes, as they pertain to different types of joints. Gaskets are typically used in flanged joints and not necessary in threaded fittings. Soldering and welding are techniques for joining pipes and fittings made from non-threaded materials or in applications requiring permanent joints, but they are not suitable for initially connecting threaded pipes.