In any application, go to the keyboard and hold the microphone key for a few seconds and a window will pop up with three icons. Choose the middle one - a T with a pen next to it. Click on that. A lined yellow pad will open on the bottom half of the screen where the keyboard was. Write on that with the S Pen and whatever you write will be converted to text in the application's text window. This works with all applications. To get back to the keyboard just hold the mike button that is now on top of the writing pad and you will get a keyboard icon to press and go back to the keyboard. Simple to go back and forth if you want to.
If you want an app to just write in and get .txt files created, Jota is free and works well. I use this all of the time as it is easier to get standard text documents than SNotes. You will be handwriting on the pad as described above, but your writing will appear as text on the Jota screen.
Also works for browsers, email - whatever the keyboard works with.