First download the DMG (installation) file from the location it is hosted. Upon confirmation that it is downloaded, it will appear on your system in the location you selected as below:
Double click the downloaded file to extract the App installer.
Double click the installer to begin the process - at this stage you may be prompted with the following dialog box, in which case simply press Open (if you do not trust the source, click Show Page to view the page where the file was downloaded).
Upon confirming the source is reliable, you can now select where to install the App to your machine along with preferences such as whether you would like to open it immediately after the installation is complete. Click Continue to begin the installation.
Once the installation has complete, the App icon will appear in the location selected for you to begin enjoying digital publications.