Tap into the song library and tap the + button, then add the title, composer, and style, and set the key the chords will be in as you enter them.
Choose a template: Blank has no barlines, while 32 Bars AABA, 32 Bars ABAC, 48 Bars, and 96 Bars lay out the form and its barlines for you.
To put the song straight into a playlist, tap into the playlist first. On iOS tap + and choose New song. On Android open the overflow menu beside the search box and choose New song (on 2026.5 and lower, tap +). The song is listed in the library as well once you save it.
For the full guide, see Create a song on the iReal Pro website.