1) Go Edit > Preferences> Advanced Midi
2) Assign each Bus to a desired Midi Port
3) Unless you plan to trigger Reason Patterns from your Main sequencer, then set your MIDI Clock sync to "No Midi input"

4) Now create a rack of your choice of synths from the Reason banks along with a Mixer
5) Go to the Midi inputs/Audio Outputs (at the top of the rack)
Note: the bus buttons: select the desired bus for your first round of assignements
6) each channel has a small arrow to the right of the channel 1 and a drop down menu appears with a list of your rack's synths - choose the module that you want to assign to channel 1
7) similarly, click on the arrow for the rest of the channels and assign them accordingly
8) now route your audio outputs either via REwire or as actual outputs into a hardware Mixer and get started....