No matter what materials you use, or how you are presenting them, it is important to always include an attribution. Linking to the original source and creator is not only proper etiquette, but enables you to find the content again if needed. Most Creative Commons licensed materials require others to at least give the author credit.
For more information about attribution review the following resources:
Use this spreadsheet to keep track of your attributions:
While Creative Commons licenses do not require you to ask for permission, sometimes it is still a good idea to ask for permission. In particular, works with a No Derivatives license may require advance permission before it can be redistributed. Most open licenses give you advance permission to do the 5 R’s with the work: Retain, Reuse, Revise, Remix, and Redistribute. However, there are still some situations in which you will need to seek additional permissions from the author/creator.
If you find content online that is free, and does not have an open license, you can link out to it, but will need permission to do the 5 R’s with it, including downloading and redistributing copies to your students. You can ask the author for permission for your own use, or better yet, request that they put an open license on their page so that others can make the same use without seeking permission in the future.
If you find content with a No Derivatives license, you still have a fair use justification to make changes in your own course. If you want to share your work more widely with an open license, and it incorporates the No Derivatives content, then you will need to seek permission from the author to use their work in this way.
Keep in mind that authors who share out educational material are already interested in sharing. They are likely to be open to your request!