H-Craft comes with a small scene-editor and a little tool to help editing the gui-dialogs.
Note that those have been written in a few weeks and haven't been touched by me in ~8 years.
So code quality and usability both have some room for improvement.

To enable the editor you have to recompile H-Craft with the following changes in IrrCompileConfig.h:
Comment out the define for HOVER_RELEASE.
Enable the define for HC1_ENABLE_EDITOR.

It's also usually easier to work in windowed-mode with the editor. Especially as you also have to dig into xml files often.
So in media/config.xml set: fullscreen="0" use_screen_res="0"
Chose any resolution you are comfortable with. Although for gui-editing it might make sense to work with 640x480 as dialogs are scaled from that base.

When you start the game now you will have a empty screen with a toolbar.

You are currently in the edit-mode. Switch to normal (ingame) gui by pressing <m>.
Also switch back to edit-mode with <m>

And you have also different application modes - editor, gui or game. 
There's also a freefly mode (with 'f') but it doesn't really work well currently so avoid that.

In short use editor-mode when you want to edit a scene.
Use GUI-Mode for gui-editing.
Use game-mode for playing the game.

<e> Put's you in editor-mode.
<s> While in the editor starts a game with the current scene and put's you in game-mode. Go back to editor with <ESC> followed by <e>.
<ESC> while in editor or freefly-mode will put you back in gui-mode

Keys <1> <2> <3> <4> let's you choose from which place to start a race.

Clicking the file-open icon will do the same as "File - Load Level"
File - Load Level will load a scene and set the edit-mode.
File - Open model file ... I think it was to add new models which can then be used in the scene.
File - Open GUI dialog will allow you to open a xml file describing some gui dialog.

Using gui-editor:
Open a xml with File - Open GUI dialog.
<g> with an open dialog will reload that dialog (for changing textures you have to restart completely).
Edit the dialogs in xml with a texteditor and press <g> once in a while.
That's it.
If you need an exampe: media/ui/menu_championship_progress_touch.xml is probably the most advanced dialog.

Using scene-editor:
There is no undo - sorry. What you do is what you get.
You can move around with the cursor-keys.
Press <shift> to move faster.
Press <ctrl> to lock the height while moving.
Select object with left mouse button.
Drag and drop by selecting with left mouse button, moving around and release the button.
When you press <x>, <y> or <z> while selecting an object you can rotate it instead around the corresponding axis.

To use a model in the scene you have to add it first. You use the Models/Nodes menu for that or edit the media/models.xml.
+ mesh is for adding a static-mesh
+ Animesh for animated meshes (.X format)
+ Light for lights (but only 3-4 are supported right now at the same time in Android)
You have to SAVE after adding new ones or they will be gone on next start.
While selecting a model in that menu you will see the corresponding node - it is not placed - just there for a quick preview.
Models can have child-models which move around together with the parent at certain distances/rotations.

To place objects in the scene you have to use the Objects menu.

There are also a bunch of special objects:
player start: The 4 positions from which players and bots start the race.
track markers: Show the direction. Also each time a player drops off the track the h-craft will be set to the last track marker reached.
TP source and TP target let you set the start and end for one teleport per scene.
finish line set's the start/finish line.

AI is a little complicated.
Basically the trick used for AI is that you enable a recorder and then drive a track. 
That's the track the AI will try to follow.
But there's also some other info the AI needs - like track-borders and track-centers.
There's a bunch of helper-functions trying to get smooth lines for those.
And lastly - in situations all that doesn't work out you can dig into the xml's for the AI and modify them with a text-editor.
And this all is very h-craft specific - the tool functions to find track-borders work for example only because the tracks are floating in empty space.
I haven't used all this for a long time, so I probably would need a while myself to figure out how to create a working AI again.
