To connect your Reaper to a digital piano, you'll need an audio interface. An audio interface is a device that connects an analog instrument (such as a digital piano) to a computer and allows both input and output of sound. First, plug your digital piano's outputs into the inputs of the audio interface using either XLR or 1/4 inch cables. Next, plug the USB cable from the audio interface into one of your computer's USB ports. Once everything is connected, open up Reaper on your computer and configure it for recording with the proper input source selected in its preferences menu. Finally, hit record on Reaper and start playing music from your digital piano!