Saving to cassette with 80s BASIC calculator, TI-74
Most probably, you will need simple amplifier based on, let's say, LM4558, but you may start even without it at all seeing if your capture device will save it properly.

You have two tasks - ensure (1) you save signal with level enough to (2) be reproduced with required level.

I also played with such implementations, and, if you choose MP3 format, use maximal settings. MP3 is lossy format, and it may reproduce modified square-like waveform which TI-84 will not capture. The best format is, of course, WAV, but the file size will be large. That's why there're programs out there to convert such files to binary files, which then can be converted back to output square waveforms using special program and target device's protocol.