Use a digital 8-track recorder if you want to record an instrumental without using other musicians. A digital 8-track allows you to use up to eight instruments to record your instrumental song. It's a good idea to start by playing the rhythm guitar part. This is the instrument that will be most prominent in the track. If you don't have a rhythm guitar part, play the instrument that will be most prominent throughout the song.
Add the bass guitar to another track while you play back the rhythm guitar through headphones. Mix these two instruments down to one track using your digital recorder.
Record the drum track while you listen to the rhythm guitar/bass guitar track through your headphones. You want to mix the three tracks together, then go back and add any solos or lead instrument you want.
Mix all of your tracks together as a single instrumental track and then upload that track to your computer either by blue-tooth or USB. Use an audio editing software product like Audacity, which is free, or any professional audio product to adjust the bass, treble, and noise levels of your instrumental track (see Resources).