checkbox save/load NSUserdefaults

I have this app with two checkboxes that i want to save when i close and load when i open but i cannot figure it out. Please can someone help me edit the code. I know need to save in NSUserdefaults somehow but i dont know how to code it.

class CheckBox: UIButton {
    // Images
    let checkedImage = UIImage(named: "checked")! as UIImage
    let uncheckedImage = UIImage(named: "unchecked")! as UIImage

    // Bool property
    var isChecked: Bool = false {
        didSet{
            if isChecked == true {
                self.setImage(checkedImage, for: UIControlState.normal)
            } else {
                self.setImage(uncheckedImage, for: UIControlState.normal)
            }
        }
    }

    override func awakeFromNib() {
        self.addTarget(self, action:#selector(buttonClicked(sender:)), for: UIControlEvents.touchUpInside)
        self.isChecked = false
    }

    @objc func buttonClicked(sender: UIButton) {
        if sender == self {
            isChecked = !isChecked
        }
    }

}