how to properly use comments-template.php

It depends on how your theme is built & we can’t be 100% sure if this is right without seeing the code but most probably comments_template is the wrong function. Take a look at the comment_form function http://codex.wordpress.org/Function_Reference/comment_form

comments_template is the function generally used to include the comments file(basically to display the contents of comments.php, though this can be changed)

Again, it’s possible for the theme to use comments_template for displaying the form & if the above doesn’t work for you, please post the code