There is an app for each type of platform including an app for Macs, PCs, Android tablets and phones and Apple tablets and phones.
Once you have downloaded the app, sign in to your institution. Visit this page if you need more information on how to sign in.
Simply click on the "download" button next to the book title, or the cover image of the book you want to download. It will download to your device and you can read it offline at any time. Books will stay on your device for 120 days unless you manually delete them.