Determine which sheet music notation software is best for your needs. Finale and Sibelius are by far the two most popular options for creating advanced and professional quality sheet music, although both are relatively expensive. There are also inexpensive options available, such as Finale NotePad, which as of 2010 is $9.95, and MuseScore, which is free. Neither Finale NotePad nor MuseScore are capable of creating particularly advanced scores, but they are popular amongst home and amateur composers.
Install the music notation software you have chosen, and start the software. The different packages typically come with a manual and example files, so it would certainly be worthwhile to review them in order to become familiar with the layout and user interface of the software.
Open a new project in the music notation software and follow the on-screen instructions to set up the staff, initial key signature, time signature, tempo, font, and any other relevant features of the sheet music you wish to create. Keep in mind that most music notation software will use common conventions to help you set up your sheet music project, but you may need to manually adjust individual score elements later if the end result does not match your intention. Try to make the initial setup as accurate as possible, however, since manual adjustments can be tedious.
Compose or notate your sheet music by dragging musical notes, rests, ornamentation, dynamics, and text notes into the score. This process will vary depending on what you are notating and your own compositional style, but you should have an idea of what you want the score to look like before you begin, regardless. Realize that it should be possible to perfectly recreate the vision in Finale or Sibelius, but some sacrifices may be necessary with Finale NotePad or MuseScore.