How to install:
Your device Must be rooted
First step for everything you do: Create a nandroid backup and make sure you know how to restore it!
I'm not responsible for anything you do with your phone
There is also the risk to soft-brick your device. In such a case, see below for ways to recover from it.
Installation of the framework:
Download the Xposed Installer APK and install it
Launch the Xposed Installer, go to the "Framework" section and click on "Install/Update"
You can verify this by starting the Xposed Installer again and making sure that the numbers in the "Framework" section are green.
Or It should like First Screenshot In this Post
Installation of any modules:
Download <module>.apk (e.g. via the builtin repository browser) and install it
Launch the Xposed Installer and go to the "Modules" section (you will also get there if you click on the notification warning you that the module is not enabled yet)
Enable the module by checking the checkbox
(note that you can install the framework and enable/disable multiple modules at once and reboot afterwards)