Install a recording program on your laptop. Before you can begin recording audio, you will need to enable your laptop for recording. While a professional production program like Pro Tools, Logic or Cubase will allow you to turn your laptop into a full recording studio (assuming you have the processing power), you can use a free program like Audacity, Ardour or Power Sound Editor for basic audio recording.
Connect your instrument or microphone to your laptop. Music supply stores carry instrument and microphone cables with 3.5 mm plugs, specifically optimized for computer recording. As a cheaper alternative, you can also connect a 3.5 mm adapter to the end of a quarter-inch stereo cable (like those used for guitars and other analog instruments), and connect the adapter to the "Line-In" port on your laptop. To connect a standard tape deck, you will need an RCA-to-3.5 mm cable.
Open your recording program and locate your "Preferences" or "Options" (usually found on the menu bar). Under your "Audio Options" or "Input Options," select the appropriate input. Depending on your operating system and driver configuration, you should select either "Core Audio," "Built-in Audio" or "ASIO." This will designate your laptop itself as the input source, rather than an external sound card or other hardware.
Click "Record" and begin playing your device. This can include speaking through a microphone, strumming a guitar or pressing "Play" on a tape deck. When you complete your recording, press "Stop." If you cannot hear any sound through your speakers while recording, locate the "Monitoring" option on your project window (sometimes under "Options" or "Tools") and set monitoring to "On."
Layer your recording. If your program supports multi-tracking, you can record multiple instruments or audio sources on top of one another by creating additional "tracks," or layers. Each track will look like its own horizontal row. Locate and select the "New Track" option (often found on the menu bar), return to the beginning of your time line and create a new recording. Repeat this process as many times as necessary.