This entry is concerned with recording on analogue equipment. There is surely a way to record a backwards guitar solo on a digital recorder, but that is beyond the scope of this particular entry.
The backwards guitar solo is an impressive psychedelic effect heard on songs such as 'Castles Made Of Sand' by Jimi Hendrix, 'I'm Only Sleeping' by The Beatles and 'Give It Away' by the Red Hot Chili Peppers. You can easily record a backwards guitar solo in your own home with a basic four-track recorder. No fancy studio equipment or computers are required.
It is assumed for the purposes of this entry that you have a general knowledge of how to record with a four-track analogue recorder. Generally when you record with a four-track, you're using the four tracks of normal commercially-available tapes, two sides by two stereo channels, to record a one-sided tape. You can put different instruments on each track, or overdub tracks successively, but the tape is only travelling in one direction. To record a backwards guitar solo you just flip the tape over. The method of how to do this is as follows:
Record your tape normally, according to the instructions on your machine, including overdubs and bouncing.
When you've recorded your last normal track and are ready to do the backwards guitar solo, do not rewind, do not return to zero, do not collect 200 currency units.
Flip the tape over.
Your tape is now at the end of your recording. Pressing play will now play your song from the back to the front. Listen to it a few times to get used to hearing the chord progression backwards.
Remember that once you've flipped the tape over, your tracks are now in reverse order as well. Your recorded track one is now on track four of the recording machine, track two is on track three, track three is on track two, and track four is on track one. So if you left track four empty for your guitar solo, once you flip the tape, the blank track will be on track one of the machine, and that's where it'll be recorded.
Cue the tape to the end of your guitar solo. That is, the part from which you'll begin recording.
Record your backwards guitar solo. Remember that the shape of your solo will come out as a mirror image of what you play. If you want a climax of high squealy notes, you have to play them first. Keep the effects on your guitar to a minimum. Reverb and delay effects will diminish the characteristic fade-in-and-cut-off sound. A little distortion and wah-wah is more than enough.
Flip the tape back over and listen to your newfound trippyness.
Repeat from step three because your first attempt probably sounded like a steaming heap of goat droppings.
NB - It's worth checking in advance which track to record once you've flipped the tape over. Usually, track one on the 'normal' side will correspond to track four on the 'backwards' side, but on some cheaper four-track models (notably Yamaha and some early Tascams) track one will correspond to track three.
This basic technique can be used in other ways. For example a single big loud strummed chord recorded backwards becomes a wall of noise that slowly fades in and grows louder and suddenly stops. You can also add backwards effects with the send function of the four-track, such as an echo that precedes the vocals. Use your imagination to come up with more ways to use the grooviness of recording backwards.