Install audio recording software on your computer. If you do not already own such software, or if the USB interface you are using did not come with any, there are readily available freeware options (Audacity being one of the most popular).
Choose a USB audio interface. A variety of this type of product are on the market, ranging from simple break-out boxes that can accept a single XLR input to multitrack mixing boards. The key is to choose what is best for your project. If you only need to record one person at a time, a simple interface will be sufficient and cost effective. If you want to record more than one person or instrument at a time, consider a USB mixer.
Connect the USB interface that you have chosen to one of your computer's USB jacks via the USB cable included with the interface. Setup differs slightly depending on what type of interface you have chosen. Follow the included instructions.
Connect your microphone to one of the XLR inputs on the USB audio interface via XLR cable. If you are not using a separate microphone pre-amp, turn on the phantom power or mic pre-amp on the audio interface.
Run your audio recording software. Prepare a track to record. This process will be different depending on which software you're using. Check your microphone while watching the level meters in the software and on the hardware if applicable. Adjust the volume on the interface or in the software until the loudest signals stop just short of peaking.
Perform a short test recording to ensure that the levels and other settings are to your liking. Make adjustments as necessary. If you chose to use a USB mixer, these adjustments can be made on the mixing board. If you chose a simpler interface, you may have to make adjustments within the recording software. When you are happy with the sound of your test recording, proceed with a proper recording.