How to manually edit Net Lines of a network
It is possible to edit the net lines of the network, but also to import net lines into the project.
Manually editing
It is possible to manually edit the network by creating, moving, and removing the lines which make up the full network. Note that using this method, the network as a whole is edited, not only a single Net Line.

The network drawing tool will be visible at the bottom of the screen.
Modes
The following modes are available.
Add

Draw a new line between 2 indicated locations in the 3D Visualization. Click in the 3D Visualization to select the start point of a new line, then click again to set the end point. The end point becomes the start point for the subsequent line, ensuring all created lines are connected.
Remove

Click on any line in the 3D Visualization to delete it.
Move

Click and hold down the left mouse button on any endpoint of a net line, then drag the mouse and release the left mouse button. The net line's end point will be moved to the location you dragged it to. If multiple net lines share the same end point, each of those net lines will now have their endpoint on that new location.
Options
To fine-tune how the editing options work, some configuration settings are available.
Connect via roads

If checked, the path between the start node and the end node is calculated dynamically to only travel via terrain with the pipes permitted property. In general, this is functionally the same as only using roads. More nodes will be placed where required, connected with net lines. If unchecked, a net line will be connected directly to the indicated nodes. I.e. as the crow flies. (This option is not available while removing.)
Repeated

If checked, removing a net line will also recursively remove any resulting dead-end net lines. This allows for the removal of long stretches of connected lines until a junction is reached. If unchecked, only the specific net line clicked will be removed. (This option is only available while removing.)
Merge

If checked, new nodes placed very close to existing ones in the same network will 'snap' to the existing node instead of creating a new one. If unchecked, a new node will always be created, no matter how close to an existing node the new node is placed. (This option is not available while removing.)
Show Constructions

If checked, these constructions will remain visible while drawing. If unchecked, constructions will be hidden so you can place nodes more easily.
Net Type

This is the network currently being edited. Only the network being edited is visible in the 3D Visualization. You can only select networks for which net line definitions exist.
Net Function

This is the net function being placed while drawing new net lines.
Merge Distance

This is the distance for merging nodes. If the endpoint of a net line is placed within this distance from an existing net node, the existing net node is used.
Node Size

This is the size used to visualize net nodes while drawing net lines. This is only for clarity while editing, and when editing concludes the net nodes revert to their normal size.