How to theme a CCK form

NOTE: CCK2 makes theming easier, for CCK2 and Drupal 6 please read here: Theme a CCK input form for CCK2

(Originally posted at http://drupal.org/node/98253)

The reason I have come up with this step by step overview of themeing input forms was that I was working with CCK created content types myself and was trying to figure out how to change the order of the fields of the input form. What I found in the forums was very technical (pointers to the API in 'developer speak') or incomplete, or simply did not cover input forms. There was a lot of help for people looking to modify/theme output, but nothing clear and concise for theming input forms.

So, as a non-developer, semi-technical, marketing/business type person, I set out to discover how to 'theme' my input forms. (All those developers and others that are better informed than I, please feel free to correct me where I am wrong!) I found there are at least two different ways to theme input forms:

1. Using the Form API (creating new .module files programmed in php and using things like 'hook_form_alter')
2. Creating tpl.php files