How far have you gotten?
In a nutshell, set the DM up like you would for Sonar, using MMC, and in Reaper,
go to options, preferences, control surfaces, and click add. In the drop down, select "Mackie Control Universal" and assign MIDI 5 for in and out. Keep the offset at 0 and size tweak at 9. Then select "Mackie Control Extender" and assign Midi 6 for in and out. Offset at 8 size tweak at 8. There's a lot more, but that's the basics.