A:
I don't believe RONR states so explicitly, but the presiding officer usually creates the agenda.
He is the one who has a knowledge of what should and must be covered.
There is no prescribed format for an agenda.
Be aware that if an item on the agenda has a time beside it, and the agenda has been adopted, that item must be dealt with when that time arrives.
For example, if the agenda says, "11:30 Adjourn," then the chairman should start the process of adjournment at 11:30, perhaps interrupting what's going on at the time.
(RONR p. 184.)
The usual agenda has these headings in this order:
It is ultimately the assembly that decides what will appear on the agenda.
They vote on the proposed agenda.
If you approach the chairman before the meeting with a request for an item to be added to the agenda, he will often agree to propose it with the change.
Items can be added to the agenda by the members before or after it's been adopted.
There are optional headings such as "Announcements" and "Opening Ceremonies."
(RONR pp. 347-358.)
BEFORE.
When the chairman announces that the agenda will be adopted unless there are additions or corrections, a member can move to Amend the proposed agenda.
The amendment requires a majority vote.
AFTER.
After the agenda has been adopted, a member can move to Amend Something Previously Adopted (the agenda).
This amendment requires a two-thirds vote.