Usually, troubleshooting involves accessing the user Library folder. There is a top level Library folder and a top level System/Library folder. Both are visible. But both are folders that you really don't need to access most of the time. Unfortunately, the user Library folder is only sort-of visible.

Using the alt key in the Go menu

If you press the alt key then the Go menu has an entry for the user Library:

Go menu with Library entry

But we want to unhide the Library permanently.

Add the Library folder to the sidebar

There is a totally useful key combo to make invisible files visible. We are now going to use this with the user Library folder.

Navigate to your user folder:

User folder

Press "Cmd-shift-." Scroll a little bit down until you see the Library folder. Here comes the trick. Drag the Library folder to the sidebar:

You can now always access the Library folder:

Sidebar with Library folder

Don't forget to press "Cmd-shift-." again to make the invisible files invisible again.

Using the Terminal

When I have screwed something up and need to access the Library folder in my backup then I needed to use the "Cmd-shift-." key combo I mentioned above to see the Library in the backup. We are now going to fix this.

Open Terminal with Spotlight or Alfred:

Start Terminal

Type or copy into the Terminal window:

chflags nohidden ~/Library/

Press enter to execute the command:

And after executing the command I see the Library folder of my backup.