Although you can write directly into your open course or resource and create it online in collaboration with others, it is a good idea to do most of your drafting in a word processing package first so you can get the structure and content right.  Alternatively you could collaborate with others online to create your content using Google docs or other similar tools.

Please note, writing for online learning is not like writing a book - for guidance on how to write your online course, see How to make an open online course.