The montage both sets the scene AND style of the film. It is supposed to reflect the blurred remnants of memory after a night of heavy drinking, whilst giving the key plot-points the audience need to get cracking with the narrative.
They are familiar scenes that the audience can instantly relate to. Their purpose is to give a clear idea of what Stan and his mates are like in a lighthearted way. They’re just a bunch of lads who are only guilty of going out to have a good time - an ethos that so many disastrous nights start on. The scenes are not intended to vilify Stan, but ground him in a realm of reality, albeit exaggerated.
However, the cheery voice of Lily on the voice-over casts a darker and more ominous shadow over the images. In introducing only her voice, this emphasises that she is the primary force of tension - it is her impending arrival that is the catalyst for the action.
The flashback shots will be very chaotic, shakey close-ups. These will be juxtaposed with the static, composed shots of Stan asleep. The sex shots will be less chaotic but will stay in big close-ups, focussing on the bodies and limbs.
The montage should last 30-40 seconds. The images will flash up from black, lasting only a few seconds at a time. There will be no diegetic sound from the scenes, the only sound will be that of the voicemail message (ie Lily speaking and breathing).
Hope thats helpful!