Here is a relatively simple and quick video guide to unlock this week's MudRunner Weekly Xbox Game Pass Rewards Quest which asks you to "Squelch 15 mushrooms." Thanks to wolfsburgraz for the recommendation.
Video here: https://youtu.be/FuxDzSWOIH8
If you're starting the game for the first time, the game will throw you straight into the tutorial. You can immediately exit the tutorial by hitting Start and selecting Main Menu. It will then try to make you do challenges, but don't accept. So, from the main menu:
1) Single Player
2) Choose The Bog map
3) Select Start New Game
4) Once you load in, you're going to want to head West/North West from the starting area, just south of the Fuel Station (Check the map with the back button)
5) Press B to remove your parking brake
6) Hold RT to start your engine and begin moving
7) Follow the road until you get to the cleared out marsh area just south of the Fuel Station (you will have to cross a large river along the way that looks like it will sweep you away but somehow doesn't)
8) Once you get to the marsh area, begin looking for mushrooms to run over. There is an image at the beginning of the video of some examples.
9) Run the mushrooms over avoiding the larger trees (you'll notice in the video I managed to get stuck between trees because the truck is difficult to maneuver)
10) Run over 15 mushrooms and you're done!
-Avoid running over a bunch of trees if you don't want to get any achievements. There is an achievement for running over 100 trees
-If you run over 50 mushrooms, you'll get an achievement for that
Hope this helps!