[support] adding "description" help text to block configuration form

I've never tried to user hook_form_alter to change a form. I'd like to just add a line or two of help text to the block configuration form on the user account edit page. Is hook_form_alter even the best way to do this? could someone just give me a pointer as to where to begin with this? Do I need to make a separate module with just the hook_form_alter in it? Interestingly, D7 will now have a few lines of help text in the block config form, but I'm on D6. thanks Neil