Connect a MIDI controller or keyboard to your computer using a MIDI cable and a MIDI-to-USB adapter. Connect the MIDI cable to the "MIDI Out" jack on your controller device, then to the "MIDI In" jack on your adapter. Connect the opposite end of the adapter to your computer's USB port. If you do not have a MIDI controller, use Reason's built-in sequencer instead.
Open Reason 4 to access Subtractor. Click "Create" on the menu bar and select "Subtractor Polyphonic Synthesizer" from the list. The synthesizer window will appear on the screen and a new MIDI track will appear on your main project window. MIDI tracks serve as digital recording spaces for the program's software synthesizers; they resemble long horizontal bars.
Select a bass synthesizer patch in the Subtractor window. Click the up and down arrows in the upper-left corner of the window to navigate between the different instrument presets. Play the keys on your controller keyboard to hear the preset. When you locate a bass setting that you like, you can either leave the current settings or make adjustments using the on-screen oscillators, envelopes and filters. For example, your two oscillators allow you to select different waveforms like sine, square, triangle and saw waves; each waveform gives you a different sound. The envelope allows you to manipulate the amount of "Attack" (the speed at which the bass responds), "Decay" (the speed at which the bass fades out), "Sustain" (the ability of the bass to remain active while you hold a note) and "Release" (how quickly the bass fades after you release the note). Return to the default settings at any time by clicking to the next preset, then using the opposite arrow to return to the previous preset.
Click the MIDI track, then click the "Record" button, which appears as a red circle directly on the track. As the recording session begins, play sequences of notes on your MIDI controller to program custom bass lines. When you finish recording, click "Record" again. If you do not have a MIDI controller, click the "Pencil" tool at the top of your screen and drag your mouse across the time line portion of the track where you want to record. Click inside the highlighted space to view the sequencer. Draw individual notes by clicking the spaces on the horizontal grid with the pencil. Each square on the grid represents a note. The high notes appear at the top of the grid, while low notes appear at the bottom of the grid.