1. Click on 'Share something with your class...'
2. Type in some information about the link, in the 'Share with your class' part. Click on the Drive icon.
3. There are 4 tabs and by default you'll be taken to your most recent files. Click on the file you want or search for it in the search box.
4. Then click the 'Add' button at the bottom.
5. This adds the link to the post. Then click 'POST'.
Your post will appear in the stream like this.
You can also look for a file by clicking on 'My Google Drive' and navigating through the folders. Once you find the file, click 'Add' as explained above.
You can also look for a file in your Starred area. This will show you just the files you've previously Starred. Once you find the file, click 'Add' as explained above.
The last option is to upload a file from your computer, but see the page on Sharing an attachment for details.
Note, in a post multiple links and files can be added. For example, see 'Scheduling materials'.