How do I turn on/off Approvals?

Go to Settings > Preferences and you will find a check box for turning on Approvals, and also one for requiring multiple approvals.

Once you have turned on Approvals in the Preferences screen, you also need to turn it on at the budget level. Go to Budgets > All Budgets, click the edit tool for the budget that needs approvals, and check the Require Approval box.