Go to the Thunderstore website and register an account.
Here's a link to their website[thunderstore.io]

In the top right there is a drop-down menu for options to login.

There are three options you can use to register an account. You can use a Discord account, a GitHub account, or an Overwolf account. Use whichever option is most comfortable with you.

In order to upload anything to the Thunderstore, you need to adhere to their Package Format.
You need these files if you want to upload your package to the Thunderstore. I created a template package folder on github so you don't have to create the folder yourself.

Go to this link: Custom Car Package[github.com]

The files are already setup for you, so you just need to download it.
Click on the green Code drop-down button and download as zip.

Extract the zip folder to wherever you want.

You don't need anything special to edit a json file. You can just use pretty much any text editor you like. Notepad is a very quick and easy one to use.
Once you have it open, this is what it should look like:

Each line is a field that Thunderstore needs in order to understand how it should display your package on the store. There is documentation for how it should be formatted here.[thunderstore.io]

The only thing you really need to change is the name. Make it the name of the car you want to upload. Replace all spaces with an underscore.

You only have to change the version_number if you are updating your car on the Thunderstore. Make sure the new number is a higher number than the previous one or it will not be accepted.

You can change the website_url if you want to link your own personal website, or you can put the link to where your car is in the community github respository[github.com] if you'd like.

The description can be changed to a short description of the car. Make sure the description is less than 250 characters.

Do not touch the dependencies field. This is here so if someone downloads your car without the Custom Car Mod installed, it will automatically be downloaded alongside your car.

As the documentation says:[thunderstore.io] the icon must be 256x256 in resolution. This is the thumbnail image that people will see on the Thunderstore.

By default, the icon looks like this:

You definitely want to edit this.

Take a nice screenshot of the car you're uploading and make sure the resolution is the correct size. After that, replace the icon.png image in the folder.

REMEMBER: The file MUST be called "icon.png". Thunderstore will not accept the image otherwise.

To edit this, all you need is a text editor just like with the json file. You can use Notepad again!

By default the README.md only says "An awesome community car"
Change this to a brief description of your car.

This is also where you want to include credits so you can credit anyone who was involved in the creation of the car.

You can also let people know which cars your car may be incompatible with, or any other tidbits you feel people should know before downloading your car.

This is what the Commander's README.md looks like on the Thunderstore page:
Simple!

If you want to do fancy effects in your README.md, here's the syntax formatting guide.[docs.github.com]

Almost done!
Thunderstore wants these files in zip folder.

First, you need to place your car file in the folder. Without it, there would be no car for people to use!
Next, select every file you want to include in the package and compress it into a zip folder!

You can name that zip folder whatever you want, it doesn't matter.

You should only include this file in the package if you are updating a car that your already uploaded to the store. Write down what you changed and people will be able to view those changes on the store if they would like to.
Again, this is only important if you are updating a car, not if you are uploading it for the first time.

Alright this is the final step.

To upload your package, go back to the Thunderstore website.[thunderstore.io] As long you are logged in, you should see an "Upload" button in the top left corner of the website. Click on that and you'll be met with this page:

This is where you upload your zip folder you just made.

Next, choose your Team name. If you haven't made one then you'll need to make one.
The team name should be your username. If multiple people were involved in the creation of this car, ask your collaborators what team name you should use. You'll want to do this if you have a lot of cars together and will be uploading them to the Thunderstore.
This team name will be displayed as the one who created the car, so don't lie! (It would be rude)

For the Communities type in "Distance". You're uploading a custom car for Distance after all.

Finally, use whatever categories that describe your car. Be sure to include the "Car" category so your car will be displayed in the Car section of the Distance Thunderstore. If none of the categories describe your car, put it under the "Misc" category.

After that, you're done! Click the submit button. If you did everything correctly, your car will now be available to download from the Thunderstore!