Cassette deck.
Sh*tty tapes.
Bounce project @ low(er) sample rate.
Cut highs/lows (or boost mids).
Run sounds through guitar pedal that is not getting enough voltage.
Set up speaker(s) in a room and re-record using mic(s).
Sh*tty tapes. lolz
Bitcrushing and other sh*t will just sound like a hifi signal... effected by a bitcrusher.
High-quality software plug-ins on high fidelity recordings will sound as such.
An OTO Biscuit is glorious but won't make anything sound "lofi" - again - just (wonderfully) bitcrushed.
I guess before I said all that I shoulda asked what "lofi" is to you... seems like it means a lot now adays. If you're answer is close to "d00d, the 303 vinyl sim..." then you probably found your answer.
EDIT: The bit about the Biscuit is just an opinion. Not an owner so when I got my hands on it I laid it on pretty thick.