captcha on custom form

Hi
I want to add captcha to a custom form only, which is hardcoded in its own php file. I Installed the plugin and tested my API keys.
I tried adding
if( function_exists( 'gglcptch_display' ) ) { echo gglcptch_display(); } ;
like this:….

			<input type="hidden" name="form_id" value="00000" />
			<input type="hidden" name="encoding" value="" />
if( function_exists( 'gglcptch_display' ) ) { echo gglcptch_display(); } ;
		</form>
		<?php	

…

to my custom form’s PHP file but the code is just printed at the bottom of the form. Being low on PHP skills, could you provide more detailed instruction as to how and where exactly to add the code so the captcha will function on my custom form?
Thank you