Install multitrack software on your computer. Free programs are available online, including Audacity. Set the software to work with your sound card. Help will be available in the program.
Connect a microphone or other audio input to your computer. Adapters may be required. Though not essential, a small mixing board can make connections easier. The mixer can remain connected to the computer, while providing a variety of inputs for microphones, guitars, keyboards or other instruments. Some microphones are now available with USB input.
Connect the sound card output to both headphones and speakers. Microphone signals may feed back through speakers, so headphones will be used in such cases.
Test the software and its ability to record signal. Depending on the software, it may take a few tries to get the configuration right, and most audio software takes some time to learn.
Block out as much ambient noise as possible. The world is a noisy place and you may not realize how much until you try to record a quiet guitar. Moving blankets, duvets and comforters can be hung over windows and doors, which should be closed whenever possible. Learn close microphone techniques to minimize the effects of sounds you cannot eliminate.