I was wondering how to make "custom fields" in Drupal 7?
No custom module is needed for this in Drupal 7.

Go to Admin > Structure > Content Types, click on "Manage Fields" by the content type you want to add the field to (e.g. admin/structure/types/manage/page/fields), select a type (e.g. "Text"), give it a name, and you're done. (There are some field settings you can change, but you don't need to.)

If you want something that does validation on the URLs, I'd recommend installing the link module, then choosing "Link" as the field widget type in the instructions above.