Create Blender Nodes Graph

To create a graph right click in Project Window. Then Create > Blender Nodes Graph.

Creating Blender Nodes Graph:

It will creates a graph with two nodes: Material Input and Material Output.
Now create a port on Material Output, so you can use it with Ctrl + Shift + Click (Auto Connect) function.
Also don't use same name for Material Input and Material Output ports.

"Ctrl + Shift + Click" Example:

Now, you can start create and use nodes.

Using Created Graph

When you click generate button, there should be created two folders named Includes and Subgraph.
Now create a Shader Graph and a material with that Shader Graph.

Open Shader Graph. Then, under Create Node > Blender Nodes Graph find your graph name.
There could be underscore (_) in your graph name if your graph starts with number.

Then, connect your subgraph sockets to desired locations.