Creating a Patch class

First, in Visual Studio, click Project > Add Class and change Class1.cs to Patch.cs

Next replace the pre-generated code with the following

using CellMenu;
using HarmonyLib;
using UnityEngine;
using UnityEngine.Diagnostics;

namespace MyFirstPlugin;

[HarmonyPatch]
internal static class Patch
{
    [HarmonyPatch(typeof(CM_PageRundown_New), nameof(CM_PageRundown_New.Setup))]
    [HarmonyPostfix]
    public static void MyPatch(CM_PageRundown_New __instance)
    {
        __instance.m_aboutTheRundownButton.OnBtnPressCallback = (Action<int>)((_) => { Application.ForceCrash((int)ForcedCrashCategory.Abort); });
    }
}

After that go back to our Plugin class and add the following line to your Load method

new Harmony("NewbiePluginAuthor.MyFirstPlugin").PatchAll();

You can now compile the plugin and when you click the About Rundown button in-game you will unlock a new rundown

Explaining how this works

The line we added to our load method means that when the plugin loads it will create a Harmony instance and apply all the patches it can find (note we supply Harmony with the same GUID we saw earlier). The [HarmonyPatch] attribute we gave our Patch class helps Harmony find it (though it isn't always required depending on how you go about patching).

When Harmony finds our Patch class it'll then look for any patching methods. The attributes we gave to the MyPatch method label it as such a patching method. The [HarmonyPostfix] attribute tells Harmony a little about how we want to do our patch while the [HarmonyPatch(...)] attribute tells Harmony what we want to patch. In this case it specifies that we want to patch the Setup method in the CM_PageRundown_New class. Our patch replaces the action of the about rundown button with our own code.

var harmony = new Harmony("NewbiePluginAuthor.MyFirstPlugin");
harmony.PatchAll(typeof(RundownPatch));
harmony.PatchAll(typeof(PlayerPatch));
harmony.PatchAll(typeof(EnemyPatch));