Basics

Node namespace specification

Firstly, to create a node you need to specify its namespace
Namespace pattern looks like this:

namespace NodeEditor.Nodes.Yournamespace
{
    // Node content
}

"Yournamespace" will be shown in "Nodes" section in right click context menu.
Also, it will become a selectable workspace in NodeEditor automatically

Node content

Since nodes have both editor-only part and build part, we need to separate them for project to build without issues.
Also, do not forget to derive your custom node from Node

public class MyNode : Node

OnDraw abstract method (editor-only)

OnDraw is an abstract method of Node class that is being used for drawing the body of the node, it's better to do with built-in API, but nothing stops you from writing your own fields

namespace NodeEditor.Nodes.NPC
{
    public class MyNode : Node
    {
#if UNITY_EDITOR
        public override void OnDraw()
        {
           
        }
#endif
    }
}

Since OnDraw uses editor-only features to draw its UI, we need to wrap this method with preprocessor directives:

#if UNITY_EDITOR
    //content
#endif

This tells Unity to not include this part of code in an actual build of project

OnInitialize virtual method (editor-only)

OnInitialize method is something like Start method in Monobehaviour. It's called once on node creation or on NodeGraph opening (workaround for fields that are not serializable).

namespace NodeEditor.Nodes.NPC
{
    public class MyNode : Node
    {
        private RealyCoolClass _swagg;
#if UNITY_EDITOR
        public override void OnInitialize()
        {
           _swagg = new();
        }
#endif
    }
}

Node configuration

At the moment, you can configure, width, color, and name (label) of the node. All the configuration tweaks happens in OnInitialize method. You can tweak config in runtime though, but I'm not sure why would you do this. Height is being calculated automatically.

public override void OnInitialize()
{
    Width = 260f;
    nodeName = "Very Smart And Kind Node";
    backgroundColor = BackgroundColor.Green;
}

By default, width is matched with label width, nodeName is matched with class name, backgroundColor is gray