Obtain a midi file for a song that you wish to notate. Midi stands for "musical instrument digital interface." These files are a way for computers to play the notes recorded by electronic instruments. Midi files actually contain information about the notes and composition of a song. Midi files can be found on the internet using various midi file search engines–and most are free to download.
Download a program capable of reading midi files. There are several free midi readers online which have gained significant popularity due to their ease of use. They share many of the same capabilities. Look for these on software sharing sites listed in the Resources section of this article. More sophisticated software is available which will allow you to tweak notes of songs, but these programs are only available for a price.
Right-click on your midi file, and select "Open With." From the list which appears, choose your selected notation software. The midi file will be read by the software, and you will see on your screen a sheet music version of the song.
Compose additional parts to the song, add lyrics or correct notes which are already present by using the available controls on your program. Click "File" then "Save" each time you make a significant change so that your composition is secure.
Click "File" then "New" in your program to present a completely blank digital sheet music page on which you can compose original work. Make sure to save after significant changes and always before closing the program. You can now open your digital sheet music files from your computer at any time.