Download a demo copy of Finale and Sibelius from their websites. There are certain restrictions for the demo copies. Sibelius allows users to use the program fully-functional for a time but not to save or print. Finale offers a simplified version called "Finale notepad," which has print and saving ability, but limited instruments and composition.
Install the programs as directed. Once the installation is complete, launch the files.
Start a new composition. In both Sibelius and Finale Notepad, a wizard comes up at the open guiding through the new score process.
Choose the instruments, key signature, tempo, etc., of your piece of music.
Begin inputting the notes for each instrument. Be sure to utilize the playback, solo voicing, transposition and other features that make notation software powerful.
Buy the full versions of the preferred program (or another program that is Mac-compatible) once you feel comfortable with the process.