Is there a common way or best practice, for creating a HTML "checkbox" form field
Yes. Use a templating system.
and marking it checked?
Yes. Use the templating system to pass the variable along, or use JavaScript/jQuery.
With that said, please, for the love of all things good, make sure this isn't an attempt to sucker someone into something that they then have to opt-out of. Any field that subscribes or commits someone to something should ALWAYS be unchecked by default. In such situations, default enabled is unscrupulous at best, and outright immoral and unethical (and more) at worst.
