Here are the directions short form:
Go to the District home page (search MISD)
Choose Mercer Island High School from the School dropdown menu.
Select "Students" on the top menu bar.
Then Select "Online Logins" from the list of options.
Select "Office 365 (OneDrive)."
Say "No Thanks" if the screen prompting to get social pop up.
Click edit and select Edit in Word ONLINE.
Be sure to sign out so that you can access your work again at school!