Launch Reason.
Click "File" and select either a work in progress from "Open Recent," or click "New" to create a blank session.
Click "Switch to Edit" mode. The default mode is "Arrange," in which you work with audio media. You need a MIDI editor interface to create a Dirty South-style pitched snare roll. Edit mode is a "Piano Roll" style editor. This is a grid interface. The horizontal axis represents time, measured in beats and bars, and the vertical axis represents pitch.
Click "Instruments" and select any snare. You will be changing the sound later.
Click in one of the grid squares on the row marked "Snare" to assign a snare to that beat. For example, clicking the sixth square along assigns a snare to beat two of bar two. Create a sequence of approximately 16 snare strokes. The last four should be closer together, to create a crescendo effect. These sequenced beats are called "MIDI events."
Right-click and select "Combinator."
Right-click in the space in the interface, click "Create" and select "NN-XT Sampler." This is a MIDI sequencer that lets you program drum loops and beats.
Click the arrow on the far-left of the NNXT interface to open the remote editor.
Right-click on the grid interface and select "Initialize Patch." This removes the default sample from the remote editor grid.
Right-click where is says "Sample" and select "Add Zone."
Double-click where it says "No Sample" and select "Add Sample." Select the "808 Snare" from the sample browser.
Drag your snare roll MIDI event from the "Arrange" window into the NN-XT sampler interface. The snare roll will appear as a series of MIDI events, each snare stroke represented by a single blocked-off square. The NNXT sampler lets you move and edit each snare stroke.
Drag the second four snare strikes down a row, to pitch them a semitone lower than the first four. Repeat this process so each subsequent four snare strikes in the roll are pitched down. This is the technique that creates the characteristic descending pitch snare roll.