In order to be able to use custom button mapping on steam, you will need to enable this feature in the Steam Client itself. The Red Boxes will show you where to click.
To do this: Go to Steam > Settings > Controller > General Controller Settings
Now enable the settings in the picture below.

To do this: Open the Menu (bottom left corner) / Xbox Button on the Controller > Controller Tab
Enable this setting:

(if you have a PS controller you can enable PS support, but I don't know if it works the same way as Xbox does)

If you don't want to set up the configuration yourself, I'll show you a way how to use the one I provided.
If you use this method, make sure to use the default layout of the game.

Click on the game in your library > Controller Layout
(Make sure your controller is turned on first)

Open the Game's page and select the Controller Layout Button.
(Make sure your controller is turned on first)
After clicking on the Button you will see this window. Now click the Layout button.


You should now see the "Load new Layout" page and from here you to the community Layout and Select the "Easy Triple Spin on LB" Layout

If you did everything correctly, the window should look like this:

Tip: If selecting the Layout doesn't work, try turning on Big Picture Mode and selecting the layout again.

Now you can use the Triple Spin just by holding the Left Bumper, whist still being able to use the normal spin on the "X" Button.

In this section I'll show you how to set up the Triple Spin on any Button you want.
For this example I'll use the default in game Button Layout and the Left Bumper as the Triple Spin Button.

Click on the game in your library > Controller Layout
(Make sure your controller is turned on first)

Open the Game's page and select the Controller Layout Button.
(Make sure your controller is turned on first)
After clicking on the Button you will see this window. Now click the Edit Layout button.
You should now see this screen:
Next you can choose the Button you want to have your Triple Spin on.
I recommend assigning your Triple Spin ("Left Bumper" in my case) on a different Button than your normal Spin ("X" in my case), because it may be hard to destroy boxes next to TNT/Nitro boxes otherwise.
Like in this example:
Now the "Left Bumper" works the same as the "X" Button.
After assigning your Buttons, open the Individual Configuration Menu and select the Settings (under "Regular Press") tab.
After opening the Menu, copy the settings below.
(You can increase/decrease the repeat rate, but I don't recommend doing that, since it may not work properly)

Thank you for reading my (first ever) Guide.
Let me know if worked for you, or if you have any questions.
Also remember to like the guide if you found it helpful!

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Disclaimer: Using this Method may attract "Purist" Players who will say, that
"It's not how you are suppossed to play the game" or that "It's a cheat/exploit"
Use at your own risk