TOPIC: Need Urgent clarification on project or phase closure

Hi All,
I have silly question on closing phase. As per PMBOK page 58, the first step of the phase of project closer is :
“Obtain acceptance by the customer or sponsor to formally close the project or phase”.
I always get confused how PM get this acceptance w.r.t closing phase/project ? Where is this procedure defined in Project charter or OPA or PMP?

Can you tell me all steps PM do in closing phase/project ? Steps mentioned in PMBOK page 58 are not clear to me .

Please help .

Thanks in advance,

Pat