From Dear ImGui git repository:

Dear ImGui is designed to enable fast iterations and to empower programmers to create content creation tools and visualization / debug tools (as opposed to UI for the average end-user). It favors simplicity and productivity toward this goal and lacks certain features commonly found in more high-level libraries.

Wrapper for ImGui.NET which in it self is wrapper for Dear ImGui - graphical user interface library. In order to use this wrapper you just need to add reference on ImGui.NET (1.89.7.1) and the mod dll itself (like with other mods dependencies, both are in this mod's archive), and add your methods for constructing ImGui windows to VSImGuiModSystem.SetUpImGuiWindows event. Another example: github.com/maltiez2/vsmod_configlib/blob/master/configlib/src/GUI/ConfigWindow.cs

Supports also Not actually (working on it, it seems I need to follow this procedure, if somebody will provide this binaries I'll inculde them in mod):

• ImGuizmo
• ImPlot
• ImNodes

This mod will allow you to make development tools like tfedit or simple throw away debug/visualisation tools for working on your mod. It would not require your mod to depend on this library if you need it only for debugging. Just wrapp all the code you dont want to ship in #if DEBUG #endif . Bulding such tools is as easy as adding logs.

If you have some field or property in your class (or just some reference type value) you want to display or edit via GUI, you can call a static mehod of a DebugWindow class that will display a widget in a debug window with specified domain as name inside a tab with specified category. Given title serves as this widget id, and calling DebugWindow methods for same domain and title will replace existing widget instead of adding a new one. Some mehod require also an int id, it server same perpouse as title. You can call DebugWindow method in any place of code, any amount of time, but setter and getter delegates will be called once per frame.

private float someValue;
private void SomeMethod()
{
    DebugWindow.FloatSlider("My debug window title", "Some sliders", "a slider", min: 0, max: 1.0f, getter: () => someValue, setter: value => someValue = value);
}

If you want to not add ImGui as dependency you can wrap its mehods with #if DEBUG #endif and leave them only in debug build

private float someValue;
private void SomeMethod()
{
#if DEBUG
    DebugWindow.FloatSlider("My debug window title", "Some sliders", "a slider", min: 0, max: 1.0f, getter: () => someValue, setter: value => someValue = value);
#endif
}

To add your own ImGui windows you can add your drawing method to SetUpImGuiWindows event of VSImGuiModSystem. This method will be called once per frame. This method should contain all ImGui methods for drawing windows and widgets. Each frame gui is fully redrawn, so this method servers as layout composer and interface logic at the same time (this is a feature of all immediate mode GUIs).

using ImGuiNET;
using Vintagestory.API.Client;
using Vintagestory.API.Common;
using Vintagestory.API.MathTools;
using VSImGui;

namespace ImGuiExample;
public class ImGuiExampleModSystem : ModSystem
{
    private ICoreClientAPI mApi;

    public override void StartClientSide(ICoreClientAPI api)
    {
        mApi = api;
        api.ModLoader.GetModSystem<VSImGuiModSystem>().SetUpImGuiWindows += Draw;
    }

    private void Draw()
    {
        ImGui.Begin("ImGui example");

        float roll = mApi.World.Player.CameraRoll * GameMath.RAD2DEG;
        ImGui.SliderFloat("Roll", ref roll, -90, 90);
        mApi.World.Player.CameraRoll = roll * GameMath.DEG2RAD;

        ImGui.End();
    }
}