Note that the ability to compose new songs is only available in the pro version.
Go to page 2 and either select the default instrument or create a new one.
If you want to change the voices in the selected instrument, click on the Page 3 button and change them.
Go to Page 2 and add a new song by clicking the [+] button. This will take you to Page R.
Give the song a title.
Tap the "Pattern Editor (real time composer)" button on the top left
Create the first pattern (bar) of your masterpiece, using the insert or [+] button or the music keyboard (shake the iPhone to get the keyboard) to add notes. See the Page R Help for more details.
Tap the "Pattern 1" button on the top right to add new patterns and arrange the patterns into steps.
With a step selected, tap on "Show in Page R" to prepare the pattern editor to play all your steps in sequence.