Stunt Rally includes a Track Editor that can be used for creating your own tracks or modifying the existing ones.

There is a 10 chapters video tutorial (version 1.7) on YouTube:

and 4 addidtional chapters (from version 2.1)

These videos (64 minutes total) cover creating a new track in detail. Even if being quite old now.
There is a more recent page describing editor features here.

Note: Creating a very simple track (or just editing an existing one) doesn't require watching all parts (at least 1,2,3,7).
Creating a good, long or difficult track reaches 2 or more hours of editing. But a simple track can be done in 5 minutes if you know editor.

0:00 gui navigation, screen resolution, graphics presets
0:51 help window, shortcuts, tooltips
2:09 basics
2:30 camera
3:20 edit modes (terrain)
5:19 edit modes (road, water)
5:48 approaches to making tracks,
7:45 quickest track - terrain align to road result (TestC8-Align)

0:00 duplicate track, set name, delete rest
0:36 terrain and heightmap sizes
1:06 brush presets (used: deform, noise, filter)
2:47 new terrain layer sand

0:00 help pages
0:30 insert basic road shape
1:33 selection, rebuild, move point, height, roll angle
2:57 align terrain to road done
3:14 bridge
3:43 bridge entry fix
4:15 smooth (flatten) brush
4:33 place car start, rotate

0:00 copy loop from other track, trk find use, rotate sel
1:17 make pipe, adjust
2:34 copy, insert point, adjust
3:16 banked terrain corner
4:00 copy jumps from other track

0:00 add water are, sizing, move, height (depth), deform terrain
2:20 add mud area
2:57 road material changing to sand (part)

0:00 adjust density of trees and grass
0:20 grass min. height
1:21 parameters description
1:54 vegetation models, and their parameters

0:00 road direction set +1/-1
0:26 mark 1st checkpoint (1)
0:35 adding checkpoints
1:40 track description text
2:07 track preview screenshot

driving tutorial track with comments on minor track errors left

0:00 objects list, inserting
0:45 height adjust, stacking
0:56 simulation toggle
1:50 select, move, delete
3:13 static building, scaling
3:47 multiselect, move group
4:15 driving on track, hitting objects

0:17 pick source track, copy terrain layers, vegetation, road parameters
1:04 sun tab, fog, rain, sky
2:08 sun light direction angles
2:43 copy, turning scenery to desert
3:16 terrain generator
4:09 scale track, terrain height
4:45 terrain layers particles, trail color
5:18 surfaces, for tire parameters
5:51 sunset, ambient color, sky

0:00 Terrain generator
2:58 New Brushes
(3:07 Top view)
4:53 Triplanar1 option and Terrain Error

All key and mouse actions can be seen in Options tab *Input/Help*, use Ctrl-F1 to show tab for current mode.

The text included on those 8 pages is a complete reference to all editor actions (and gets translated to your language if supported).

This text here is only a quick reference to most common actions.

F1 or ` - GUI Options toggle

Tab - Camera / Edit mode toggle (current mode is shown in left bottom corner: Cam, Edit or Gui)

• Terrain
  • D - Deform
  • E - Set Height
  • S - Smooth (flatten)
  • F - Filter (remove bumps,noise)

• Road
  • R - Edit Road
  • Q - car Start Position and area

• Extras
  • W - fluid areas (water or mud)
  • X - objects (dynamic or static)

Ctrl-F1 - Press to show help page for current mode.

F4 - Save track (after this track is ready to be played in game)
F5 - Reload track (one time undo)

V - toggle Vegetation (hide it, for more fps when editing road)
G - toggle Fog (when editing from distance)
I - toggle Weather (rain, snow)
Alt-Z - toggle top view camera

B - Rebuild road (after terrain under it changed, etc.)
F8 - Update (after changes made on Terrain/Vegetation tab)

U - align terrain to road tool (in road edit mode, for selected segments)
C - toggle on/off dynamic objects simulation (in objects mode)
F7 - enter/leave Preview camera

Approaches to making a track:

1. Make great terrain and place road in it (difficult)
2. Make cool road and decorate terrain after (easy)
3. = 1+2 Mixed mode. Edit terrain and road freely.

A list of short steps that describe creating a new track on a high level:

Save frequently (F4), after each step or change. When made a mistake hit F5 to reload last track state.

• 1
  • load an existing track - that has the most of desired features already set up (eg. has good terrain layers, vegetation, terrain heightmap etc.).
  • enter new track name, press button: new (duplicate)
• 2
  • on terrain tab, choose area, make new flat terrain (pick heightmap and triangle size) - when making new terrain shape
  • edit terrain (deform up/down or smooth)
  • it's good to place car start now and test terrain scale in game by driving
  • optionally edit terrain layers
• 3
  • draw initial road shape - inserting road points from top view
  • edit road points (make bridges,etc), adjust roll angles, optionally use align terrain to road tool
• 4
  • setup car start position and its area (in Edit mode press Q and Ctrl-LMB to place it at cursor) - after this track can be driven in game
  • choose road direction (+1 or -1), add road checkpoints (spheres, chkR), mark 1st checkpoint - after this track timing works
• 5
  • adjust vegetation amounts (grass, trees)
  • optionally edit vegetation layers and other parameters on vegetation tab
  • enter preview camera and make a track screenshot
  • drive the track, go back to editor and make changes to road / terrain to get the desired diving experience (this is quite important and best to repeat it often, starting early and after each major change)