Lobbying Congress can be something as simple as writing a letter to your members of Congress, or as complex as hiring a large (and expensive) lobbying firm to do it for you, and just about everything in between.
Many organizations lobby Congress, and have a committee set up to do that. What you do will depend on how many people you have involved and how much money you have. Letter-writing campaigns, when well organized, can be quite effective.
A full answer to this question, however, could constitute an entire term of a university subject.