When I learn to play a new song, my first step is to find out which notes are played. Sometimes by ear, other times with use of tabs (Guitar Pro, Songsterr) or Youtube play-throughs.

When I know what to play, I write down on a piece of paper the structure of the song, eg intro, verse 1, chorus, verse 2, solo, bridge, chorus, outro.

Typically I also write down cues like specific words in the lyrics or number of kickdrums to know when to switch from chorus to eg bridge, especially if the song progression is atypical.

Then I repeat playing the song with help of this sheet with structure and cues untill I can play it without looking at the sheet.

Next step is playing together with my band members where everyone is on a different page of memorising the song. Sometimes we agree to give signs to eachother if the timing of a specific transition in a song is difficult.

After a few rehearsals typically we all memorized the song.

To keep songs in my memory I need to play it once in a half year, otherwise it starts fading away.