There's basically two ways :
a) most new-ish audio interfaces have Direct Monitoring - which routes the monitoring out before it goes through your DAW. In this case you'd turn Reaper's monitoring off for the track(s) currently being recorded.
b) if you have a reasonably fast computer and a reasonably good audio interface, you can probably run it at low enough latency to monitor through Reaper with a delay so small it won't be a problem. This is done by setting the audio interface's ASIO buffer to a very low sample size like, say, 64 samples (sometimes expressed as milliseconds). The ASIO buffer controls the "rate of feed" of audio going in to your DAW, and can be set both according to the ability of your computer to process and to the capability of your audio interface and its drivers.
A third option is to use a project setting with a higher sample rate - double the project sample rate (ie from 44.1khz to 88.2khz) should almost halve the latency ... the downside being that it makes your CPU work twice as fast to process it.
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