The Apple II has built-in support for reading/writing data to cassette tapes. We can use this same mechanism to transfer data from a modern computer to an Apple II.
This tool converts Applesoft BASIC source code into a .wav audio file. The sound waves encode bits of data: a short pulse is a 0, and a long pulse is a 1.
Install Rust, and then run:
$ cargo install bas2wav
To convert basic-program.bas to basic-program.wav, run:
$ bas2wav basic-program.bas
Power on the Apple II, and press ctrl+reset to get to a BASIC prompt.
Plug in an aux cable to your laptop's audio output, and the Apple's audio input (right hand side).
Type LOAD on the Apple, and press return. The Apple is now "listening".
On your laptop, play the audio file basic-program.wav using a web browser or
music player. This transfers data from your computer to the Apple II.
And that's it! Once the audio finishes playing, you should hear a "beep" from
the Apple II, and be returned to the BASIC prompt. You can type LIST to see
your program, and RUN to run it.