Connect a monaural or stereo microphone through a 3.5mm male plug into the microphone input of your sound card or laptop. For CD quality, use a stereo USB audio interface with XLR and 1/4-inch jacks for studio microphones and line-in/instrument connection.
Load and start an audio editor and recording program on your computer. This can be free programs like Audacity, or bought like Cubase, which are typically packaged with USB audio interfaces (see "Resources").
Set the recording device preference in the I/O section or in the device toolbar of your software. This is where you select mono or stereo, as well. Set the input source for the selected device, such as microphone or line in. Adjust the input level on your software mixer. Click the software record button to record sound. To stop recording to listen back, click the button with the square.
Use a USB cassette-to-CD or MP3 converter device for cassettes. This comes with easy-to-use software to guide your process. Or use a component cassette deck. Use two RCA-to-1/4-inch mono adapter cables between the output channels of the deck and the 1/4-inch line-in jacks of a USB audio interface, then run an editor/recording program.
Use a USB turntable with its own easy-to-use software plugged into your computer for 78/45/33-1/3 RPM analog disks, or use your existing hi-fi with a USB audio interface. The second option uses the stereo to preamplify, and stereo RCA outputs with the same adapter cables described in Step 1.
Play back your media and record to hard disk through whatever conversion software you choose. For CD files, choose .WAV file output, convertible subsequently to MP3. Or choose MP3 format to load directly onto players, USB sticks or SD cards. Experiment with Kbit/sec settings for a happy medium between quality and file size.