Create custom write pages manually?

Hi folks.

I′ve using Custom Write Panels for a while. It fix nearly perfect for my need. But I want to make custom write pages by myself, manually, and not with a plugin.

Custom Write Panels works fine, but I need more customization bulding the write pages. For example, put custom fields text boxes beetween title texfield and post textfield, and show only the children categories of one category in the category list, instead of all categories.

I′ve searched the codex looking for documentation that explain a way to do this, but I found nothing. Maybe I look in the wrong way in the codex, or something else.

Someone knows where I can find documentation explaining how to create custom write pages manually to add them in the admin folder and menu?.

Huge of thanks to all of you.