It will open the area where you can choose elements to add.
Drag and drop the Sub-graph box to the graph table, then close the TOOLBOX area. Your sub-graph will appear at the point where you dropped it.
Zoom in with the button "+" at the bottom-right corner. You can also use mouse wheel or your fingers on a touchpad.
Drag the table for your convenience.
Open Settings with three dots at the top-right corner of your new sub-graph. You will see IO SETTINGS ("input-output settings").
Add inputs and outputs that you need for this sub-graph (1). If your sub-graph has to receive multiple inputs of the same type choose the Multiple option (2). Outputs are multiple by default.
Write a name for the input (or output) and choose where it must appear on the sub-graph (3). The center option will place this input (output) at both sides of the sub-graph.
Choose an appropriate Socket type for the input (output) (4). You will be able to connect nodes and sub-graphs through compatible (similar) sockets. If you choose AnyData from the Common socket group, this input (or output) will connect to any type of sockets, but it will change its own type with the first connection accordingly.
To delete an input or output, press the trash bin icon next to it (5).