Not sure what you're planning.
If you are thinking of .htaccess on a directory containing the forms, then using HTTP_REFERER as a check at the script level then you are open to one serious problem: HTTP_REFERER spoofing.

Your scripts will still be accessible by someone spoofing their environmental variables. It's not that hard to do. It's like hot wiring a car. Maybe not everyone knows how, but enough do that it's a problem.

So to make the scripts a little more secure: .htaccess with password verification at the form level might be a good idea.

Then to keep the poor user from having to input the password each time combine with cookies. It's a kind of double login which might be worth the effort.

Hope this helps, Claude