If you are having Better UI 1.x already in your project, follow the Upgrade Guide first.
To install Better UI, simply download it from the Asset Store with Unity and press the import button.
Then import everything (you may leave out the Example folder, if you don't want to check out the example scenes).
After importing and recompilation of the project navigate to "Assets/TheraBytes/Resources" if it exists and check if there is a file called "ResolutionMonitor" and a file called "Materials".
If the folder or either of the files do not exist, navigate in the menu bar of Unity to "Tools -> Better UI -> Settings -> Ensure Singleton Resources". Now the files should have been created.
Note: The files are also created when accessed the first time. So, this step is not required but for version control it makes sense to commit these files with the initial Better UI installation commit.
The Materials settings are generated at startup and are not important to configure. It might become interesting when you want to write own Materials (see here).
The Resolution Monitor defines which kinds of screen types / orientations you want to support as well as the reference resolution. Per default there is a Landscape and a Portrait setting with Full-HD as optimized resolution. If you need a different setup, open it and adjust the settings accordingly (more information here). To open the Resolution Monitor, navigate to "Assets/TheraBytes/Resources" and select "ResolutionMonitor" or go to "Tools -> Better UI -> Settings -> Select Resolution Monitor" from the Menu bar.
The Resizing-features of Better UI only works if your Canvas Scaler is set to "Constant Pixel Size". The other modes would result in unexpected final sizes. If you want to use a different Canvas Scale mode (which is actually not necessary, if you work with the Snap Anchors Tool) you would need to expand the "Size Modification" sections of any Better UI Element you are using which has resizing options and remove all Size Modification Entries there.