Headers and page redirecting


I am creating a simple contact form with field validation to make sure
users are entering valid data. I am posting the form data via CGI to the
same page so it can run the validation, if there are errors the page is
the same and the fields are highlighted where there are errors. All that
works fine. Now I am trying to redirect the user to a new page depending
on certain conditions determined by their post data. HOW? I have found
stuff like:

cgi = CGI.new(“html4”)
cgi.header(“Location” => “/some/other/page”)

But it doesn’t work? Is it because headers are already sent or
something? I feel like what I am trying to do should be relatively easy.
Thanks in advance!