Set up your computer, and tune your instrument. Open the program you will use to record the instrumental MP3, and set an option such as "built-in microphone" as the input. Of course, if you have a USB microphone you can use this too.
Hit the "Record" button, often pictured as a red circle. You don't need to begin playing immediately because you can edit the track later on.
Begin playing, at a medium volume. When finished, press the "Stop" button, or the "Record" button again, depending on the program you use. This will stop the recording, and you can listen to what you just played.
Look for distortion as you listen to the track. Distortion appears frequently in instrumental recordings because you can easily "peak the meters." In some programs, such as Garageband, you will see a red light indicating that you have exceeded the capability of the volume meter. In other programs, you can see it on the recording because the sound waves look thick and fill up the space. This causes the distortion you hear when you listen to the track, which means you need to re-record.
Listen carefully for any mistakes, which occur often in a solo instrumental MP3. While you can edit the track by splicing and re-recording certain parts, a beginner will have trouble doing this. Most of the time, re-recording represents your best option if you have made any serious errors.
Delete the beginning and end of your track, where you aren't playing. Click on the places where you want to split the track. Then choose the option under an editing menu you can select to split the track. Delete the unnecessary parts.
Finish editing your track. You have many options here---enhancing the tuning, adding effects or changing the volume, among others. You will find it helpful to take a look at the equalizer and play around with it. It takes a long time to learn how to use an equalizer, but you can attempt to make your track better just by experimenting with it.
Export your track to MP3 format. Select "Export to MP3," or something similar, and your instrumental MP3 will be ready in just a few seconds.