How to easily switch between Skyrim and Something to take notes on?

I have used a web based editor in the browser within the steam overlay for this. You could use something like gist, evernote, markable.

Press shift tab to bring up the overlay.

Skyrim doesn't handle switching between applications very well. At all. Roughly 50% of my alt-tab attempts with Skyrim end in a crash to desktop. You could switch out of Skyrim with alt-tab (or similar key combo, depending on your operating system), but I don't really recommend it.

If you're not too afraid of mods, you could use Take Notes - Journal of the Dragonborn. It adds an in-game journal which could be used to take notes within the game. It also allows you to export your journal entries to a text file.

Play in windowed mode. After starting Skyrim, go to options and enable the windowed mode setting. The windowed mode plays much better with alt+tab compared to playing fullscreen.

Unfortunately, you will now have a non-borderless window decorated with the Skyrim titlebar and the max/min/close buttons of a normal Windows' window. I have yet to find a way to launch in borderless window mode without mods.

Why not use a tablet while you play?

Thats what I do

Overwolf is an overlay system similar to steam but offers a number of addons including a notepad that i use for the similar purpose, when playing some games.