This app was mentioned in 2 comments, with an average of 3.50 upvotes
I use a minimal and free fretboard app that basically quizzes you on where notes are. You can set it to ask you to identify the note at a particular fret, to give you a note and then you find all the frets, or to give you a note and you find just one fret.
The one I'm using is particularly useful because you can configure the number of strings and where they start.
https://play.google.com/store/apps/details?id=lt.joru.learnguitarnotes
Don't know if you have an Android smartphone, but I remember this small app being quite easy to use and helped me a little bit on training:
https://play.google.com/store/apps/details?id=lt.joru.learnguitarnotes
Other than that, using a piece of paper is also a good idea. Just keep on writing and writing all over again. And thanks for reminding me that I need to come back and practice this more, lol.