If you want to make your own patcher google Harmony. If you want to go for a low effort approach use dnSpy or ILSpy to decompile a class, modify it, and recompile it.
I personally think dnSpy is much better than ILSpy but it's preference.
EDIT: if you use dnSpy or ILSpy you'll come across common and easily fixable errors, just read the forum to learn how to fix them.