First you'll want to create a group for "Approvers" - always a best practice to assign permissions/privileges at the group level to make it easier as people come and go!
You'll also need to modify a campaign definition (or create a new/copy of an existing) that has approvals enabled and uses your new group to grant the "Approve" permission on campaigns created with that definition. (Note that you can't apply approval to existing campaigns, only ones you create with the "New" button that are based on this new/updated campaign definition.)
You might also need to update the definition to include the approvals page if you've previously removed that.
The last thing you need to do is decide how much enforcement you want. By default, you can't execute without it being approved, but even after it's been approved users can still change things; essentially you get a paper trail and can generate a diagram document at the time of approval, but it doesn't make those last minute things stop you from getting a campaign out the door. The alternative is to turn on the business context-level setting to "Open Approved Campaigns as Read-Only", which does exactly what it says: doesn't let you make any changes to an approved campaign. There's a caveat, though: you can't execute a campaign that's read-only, so someone with Approver permissions would have to schedule and/or execute it.
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