Setting up ASIO Drivers
ASIO drivers are necessary for live VST effects monitoring. Enabling ASIO drivers is very easy in Reaper.
Go to the Options menu and choose Preferences. Then click on Device located under Audio (if you don't see Device, double click on Audio to expand it). In the Audio System box, choose ASIO. If you don't see ASIO in the list, then your soundcard probably does not support ASIO. So you'll need to download
ASIO4ALL so your soundcard can use the ASIO drivers. I don't really have the ability to help with any installation problems since both my soundcards have ASIO drivers already, so you may need to post on their forum or ask someone here that uses it for help.
If your soundcard is a multiple input soundcard, you can select the range of inputs and outputs that you want to use. So make sure that the full range is chosen in this section so you can utilize all of your soundcard's connections.