I recently downloaded an app off of an unknown site because it was too large to install off of Google Play. It downloaded as an apk and now I do not know how to access it.
First, avoid downloading APKs of the apps, always install from Google Play. To access the downloaded APK, open your phone's file manager or install one (like ES File Explorer). Navigate to the path where you have saved the APK file, and just tap the APK file. It will take you to the installation process.