Tape music, music concrete, is a precursor of sorts to electronic/synthesized music. Tape manipulation can include but is not limited to, the use of tape loops, reversing, overdubbing, re-splicing, degrading/destroying the tape etc. Or using as source material (field recordings/pre-recorded stuff)
It's kinda trial by error process, i don't think there are any rules. if you wanna try it out start fucking with cassettes, jam the gears/unspool the tape and play it, whatever.
Other things that might help you are watching visual examples (youtube should have a ton of shit), check out Joseph Hammer, Aaron Dilloway, Cluster & Eno and read up on musique concrete. But more than anything, i suggest just unspooling a tape and fucking with it. Hands on/Trial and error is the best way (IMHO)