Note: These instructions are the same on a Mac or PC, though there might be minor variations between the two platforms.
1. In Reaper, select the Track menu and click "Insert virtual instrument on new track..."
2. From the Virtual Instrument window, select "VSTi (or VST3i): Your Instrument (Spectrasonics)". If you're using a Mac, you can also select the AUi version.
When you see the "Build Routing Confirmation" pop up, click No to use your instrument as a Stereo instrument, or you can also click Yes to setup your instrument as a Multi-output instrument. Clicking Yes will create additional MIDI tracks in your session for each of the outputs for your instrument.
Note: Keyscape can only be used as a Stereo instrument.
3. Your instrument interface will appear. In the screenshot below you'll see that the Omnisphere instrument track has been created in Reaper's Mixer.
Here are some downloadable templates which you can use in Reaper: