How do you select multiple measures

Lots of ways: https://musescore.org/en/handbook/selection-modes#range-select

Also see the section on lines, which explains how to lengthen or shorten a line fter entering it. but indeed, the best way is usually to preselect the range, and any of the many ways documented in the Handbook can work. Note these are all pretty standard - they aren't really unique to MuseScore. For example, pretty much every program in the world has the notion of clicking something and then shift+clicking something else to select everything between those points.