The trick for this part is to add the Security Assist random password binding to the page first. Then you will be able to select this value as a binding, when you select it you will configure the settings of the random string it creates.
Once you have got it all setup you will want to adjust the code where this value is set for the item, you will want to check that it is the correct type of item, unless it is static. If you need some additional help with this or more advice let me know.