you highlight a bit of text, and that should result in a pop up of options, one of which is the private notes one. It does not always work that well. You are limited in how many words you can put in.

Here is a link to an exellentbarticle on how to do this.

https://medium.com/new-writers-welcome/how-to-create-private-notes-on-medium-fd5b804aef99