Appending to a new file:

If you want to add them to another Blend File.

You press File > Append then you navigate to the Blend File of the Nodes

You click on the Blend File and select the folder NodeTree, and you select all the nodes or only the ones you need [similar to normal file select].

Then you visit the Compositing Tab in your file, press Use Nodes in the Compositor, and then you can add them with Shift + A and Search (it is similar to every other Node Editor).

The Image Output from the Render Layer or Image comes into the Input from the Node and the Output to the Composite and/or the Viewer Node (in Search under Viewer or for Image with Image, and you select your Image).