I need a set template for some of my Wordpress posts but only for a few. Can I simply design a template in html and insert it into a Wordpress post? I have tried this and the editor keeps changing my code.
You can, just as long as you dont use the editor when creating/editing the page. To do that you will have to hardcode every change to that post.
Do you mean disable the WYSIWYG Wordpress editor, create and new post then simply paste in the HTML code?
Go to the WordPress forum http://wordpress.org/support/ and check their previously ask questions or just ask them, they're always helpful and should be able to solve this for you quickly
Thats exactly what I mean. Alternately research on the official WP site how to create page templates. This is a clone of your page.php theme file which you would then insert your HTML into and set the relevant page with that template. As far as i'm aware this is only available in pages, and not posts.