I regularly have difficulty with this despite reading the handbook carefully. It seems to be a topic for which the Musecore approach is counterintuitive for me. The concepts, in particular the meanings of the beam symbols presented in the handbook, are hard for me to grasp.
Perhaps an example will illustrate. I have 4 quavers that have been beamed together. I would like the first and last notes not beamed but the middle two to keep their beams. How can this be done easily? The way I try is: pick the second note and break its beam to the first - this works. Now pick the third and break the beam to the fourth - can't do it. Instead I have to pick the fourth and break its beam. Why?
Suppose I unbeam all 4 notes and now try to beam the middle notes. How can this be done easily? I pick note 2 or note 3 and click the first beam symbol - nothing happens. I need to add beams to notes 2 and 3 successively and I need to remember the beam settings for each note since they are not shown in the score. I find this very hard to do and to remember.
I would really like to see a detailed tutorial on this subject which explains the concepts and procedures better than in the handbook.