A notice of default is the last opportunity you give your counterparty to still fulfil its obligations. A formal notice must meet three requirements:
Written: a formal notice can also be sent by e-mail under circumstances. Oral notice of default is not possible.
Summons: a summons to your contracting party to still comply with a specified obligation. In the notice of default, clearly describe which agreements still have to be fulfilled.
Notice of default: a reasonable period given to your contractual party to comply.
Failing to comply within the specified period puts your contracting party in default and may give rise to claims for damages (among other things).