To create an empty segment, in order to begin composing something new, you need to use the pencil tool (which is normally the default active tool when you first start Rosegarden). Click on the pencil on the toolbar, and then click at the point where you want the segment to start (at the correct height for the track you want the segment to be on, and at a distance across the editor window corresponding to the correct time) and drag rightwards until the segment is the right number of bars long.
You can also create segments using the Select tool, by clicking and dragging either with the middle mouse button, or with the left button and Ctrl held down.
If you want to draw a segment longer than the visible portion of the canvas, the canvas will scroll automatically once you reach its edge. If you keep dragging past the end of the entire composition, you can keep dragging, and will push the end marker right along with you. (This is a welcome contrast to earlier versions of Rosegarden.)
Usually each new segment starts on a barline and extends bar by bar, but this snap-to-grid effect can be prevented by holding down Shift while clicking and dragging.
New segments are created in a yellow colour, with a treble clef by default. You may wish to jump ahead and have a look at track parameters to explore other possibilities.
If you wish to draw a segment that begins in the middle of an existing segment, hold Ctrl and Alt when clicking. The track will automatically expand to accommodate this new segment. This is especially useful for some situations involving music notation.
Once you have created a segment, you can then begin editing using the matrix, percussion matrix or notation editors. To use one of these editors, either double-click on the segment, or else click with the right mouse button and choose the editor you want from the context menu.
If you are intending to record from a MIDI device or audio input, you don't need to create a new segment to record into first -- each separate recording always goes into a new segment, created automatically during recording.