Head over to your profile settings and there should be a checkbox called "Use rich text editor." Uncheck that to disable it.
If you disable RTE and use HTML on your page like what myself and others have shown you in this thread, it should work as intended. However, if you reenable RTE after making the changes and try to change that HTML-modified page with RTE on, it might break the HTML.
I'm not 100% sure, but just to be on the safe side, I would recommend making ALL of your HTML changes to a page in one fell swoop before turning RTE back on and continuing on the other pages. If you leave that page untouched after turning on RTE, you should be fine to continue using RTE so long as you don't try to edit the page with HTML.