At first, you have to get Unity 2017.4.9f1. I also recommend blender, Gimp and Visual Studio.
After that, you have to download the modmaker package under here. If you want you can download the other package, too. Do the steps written there.
Now create a new Unity project. It has to be 3d.
The first thing you have to do is deleting every scene except the one you want to use. Also delete every camera.
Then create an empty gameobject and tag it with "startline".
Create another empty gameobject and tag it with "finishline". Give it a box collider and set "Is trigger" to true.
Then make a map. There is are a ton of possibilities there. If you want, you can also create scripts, but have a look at the official modding guide for that.
Now you can either drag'n'drop obstacles into your scene or create your own ones. But be sure to have colliders on all of them.
When you're done, click "Tools" at the top and then "Export mod". Fill in the gaps and export it into your mods folder.
Thats how to create a mod!
Image source is the title image of steam.