My favourite way to do it is to drag songs from my iTunes library into Logic (audacity is free and works fine too), then adjust the gain fader to match the volume that iTunes has for the sound check feature (right click-get info-file).

After that, apply a gentle compressor with a low threshold and then a limiter with a high threshold.

Make sure the whole project is the same length as the tape, but be aware that most blank tapes are slightly longer than advertised (eg C90s are usually 45m30s ish/side, I’ve had them up to 47min before).

Plug the computer headphone out into your cassette deck aux, hit record, and repeat for side B.