Open your preferred music production program, for example Reason or Logic. Double-click the desktop icon. This will open the last session you worked on. If you want to call up a different session, click "File" and select "Open Recent." Click "Open New" to start a new session from scratch.
Open the "Arrange" window. Typical production programs have an interface for arranging and an interface for mixing. The method of selecting each interface varies according to which program you use, but typically you need to click a single tab. For example, in Ableton Live click the circular tab with the three horizontal lines to open the "Arrange" window.
Click "File" and select "New MIDI Channel." Although you want be composing in the "Arrange" window to integrate your pattern into the song, you'll need to export it to the window once complete.
Set a tempo. Where it says "120 BMP" at the bottom of your screen, type in a new tempo. 120 is the default beats per minute tempo, but you can increase or decrease the tempo to your preference. If you're not sure what tempo you want, leave it at 120 and change it later. When using MIDI only, you can make tempo alterations after sequencing. But make sure you have the tempo set before adding any audio tracks.
Assign an instrument to the MIDI channel. Open the instrument list. The method varies slightly, depending on program. Typically the instruments are located under a "Sounds" or "Media" menu. For example, in Logic click "Instruments," then choose from a category such as "Pianos" and select your preferred piano.
Open the pattern sequencer. On the left hand side of the MIDI track, click "Plug-Ins" and select the pattern sequencer for your program. Each program has a different name for its pattern sequencer. In Logic, it is called Ultrabeat and in Reason it is called Matrix. All pattern sequencers have a grid interface. The vertical axis represents pitch and the horizontal axis represents time, measured in beats.
Click "Play" on the pattern sequencer. This configures it to repeat in a loop. Plot your notes on the grid. Click on a grid cell to add a note to that region. For example, click on the first grid cell on the bottom row to make the lowest note the first in the bar. As the sequencer loops, you hear the extra notes that you add. To delete a note from a cell, click on it.
Click on "Pattern" in the bottom-left of the screen and drag it up to the MIDI track to export it.