Select the second track, and rename it to "Percussion" (or something else, if you prefer).
In the "Instrument Parameters" portion of the toolbox on the left side of the editor window, check the "Percussion" check-box. If you do not see the "Instrument Parameters" panel, select → from the menu.
Select Play on the transport for just a second, then stop it. Rosegarden synchronizes the re-assignment with FluidSynth
Press F3 on the keyboard, or click on the "Draw" tool that looks like a pencil.
Create a one-measure segment in the second track, then right-click on the segment, and select .
The percussion matrix editor is like the matrix editor, but each pitch is labeled with the instrument it triggers.
Experiment with the different instruments and pitches to find a pattern of four quarter notes that you want to repeat in each measure.
When you finish with your pattern, close the editor.
Select the newly-created percussion segment, then choose → from the menu.
Move the transport playhead to the end of the first measure. First move it to the beginning, then press the "fast forward" button to advance it to the end of the first measure.
Confirm that the second track is still selected.
Choose → a few times, so the percussion track fills up the same space as the bassline.
