Instructions
Head to Darnassus, Stormwind, Ironforge, Orgrimmar, Thunderbluff or the Undercity and find that city's Guild Master.
Pay the Guild Master 10 silver pieces to receive a Guild Charter. Have a name ready for your merged guild.
Find 10 people to sign your charter. Your own signature counts, and you can use people in your current guild as well as the guild you're merging with. Anybody that signs the charter will automatically join the new guild when it is created.
Turn in the charter to create the new guild. Everyone that is in your original guild and the guild you're merging with can join the merged guild.