You can easily add a breakpoint to your code by clicking on the grey column to the left of the line of your VBA code where you want execution to stop. A red dot appears in the column and the breakpoint code is also highlighted in red.
You can add multiple breakpoints throughout your code and resuming execution is achieved by pressing the "play" icon in your menu bar. Not all code can be a breakpoint as variable definition lines, the first or last line of a procedure and comment lines cannot be selected as a breakpoint.