I love multi-track cassette recorders!
I've had a Fostex X-12 four track recorder for about 8 years, and I actually just ordered a Fostex XR-3 the other day(it has 'varispeed,' which I hope to use for vocal pitching like prince, or on 'don't talk to strangers')
I pretty much record most of my songs to my computer first, like all of the midi keys, drum machine samples, guitar, bass, vocals, etc, etc, whatever. After that, I basically use the X-12 as a tape hiss generator, since I'm not recording to tape yet at that point of the song -- the audio is just going through the machine without being recorded-- and since i'm not recording to the tape yet, I can turn up the track levels on the multitrack recorder really high, or turn up the gain on the multitrack input(which can also give you some great distortion/noise) to get more hiss.
So basically I run an individual instrument track from my computer, to a chorus pedal, to the four track, and then record it back onto my computer with the new hiss. Then once I've ran each instrument track through the hiss, I record the song as a whole onto the cassette tape, usually 1-3 times, depending on how much warble or hiss seems appropriate for the work.
Sometimes I'll just record lots of tape hiss onto my computer with whatever recording program I'm using, giving the hiss it's own track, and then turn up the volume until it gets loud enough in the mix
Also, using unbalanced quarter inch cables can add a little noise, i've heard, but I haven't had a chance to use mine yet
Lastly, experimenting with mixing can sometimes help, because you can highlight certain frequencies or textures. Like, a lot of Ariel's lo-fi sound comes from his mixing, at least imo, it seems. He's such an awesome mixer! Sometimes taking some of the (lower) high frequencies/treble out of a recording can make it sound lofi. The thing w a lot of Ariel's lofi work is that, idk, the mixes don't sound dull, you know? Like, there's still a lot of warmth and character to it, like, he's not just taking out some of the treble, it's pretty professional, even though it sounds 'lofi' -- His mixes are art!!
Idk, I don't really have any other ideas or anything. . just keep experimenting?
here are a couple examples of songs that I've made that way. One of them is kinda 'noisy,' i guess, but most of my work is just really hissy https://soundcloud.com/joshuajcotterino ... urden-down