Connect the RP150 using the 1/8-inch stereo mini plug cable to the headphone jack on the rear of the unit to the light blue input jack on your sound card. Use the sound card's software mixer or your recording software mixer to adjust the level of your guitar signal.
Set the "Amp/Mixer" button on the back of the RP150 to the mixer setting ("In" position) to optimize the unit for direct recording purposes. The button is between the input and output jacks.
Bypass the sound card by connecting the RP150 with a USB cable. Connecting this way will activate a USB codec, which you can select for the sound source for your software. Connection in this manner bypasses a digital-to-analog conversion to get to the input of the sound card, and an analog-to-digital conversion in the sound card itself. Fewer conversions generally equates to better sound quality.