First you will need to purchase an audio cassette
I recommend new cassettes.
Advantages: produced recently, good value for money, no trouble with sound
I do not recommend cheap cassettes from China. The recording will be of very low quality, these are suitable only for recording speech, but not music.
You can buy an old cassette, but you definitely need to check it (record something and listen to the sound quality). It happens that even cassettes from good manufacturers deteriorate very much due to poor storage conditions. There's always a lottery with old tapes.
Recording…
You will need a device with the ability to record to an audio cassette. A special cassette deck is best suited, which will provide the best recording quality as it has some additional settings for recording. You can also use a cassette audio center or a tape recorder.
The music source should be connected to the input channel of the recording device. It is best to use a computer with a high-quality sound card and high-quality music (I recommend 44100 16bit wav). You can also use higher quality files, but as a rule, the specified quality is sufficient.
It is also possible to use any other device capable of playing music as a signal source.
Insert the cassette into the recording device, rewind it to the very beginning, adjust the volume and additional recording parameters, if any, on the recording device. Press record by pressing the "Rec" button and turn on the music playback on the signal source. I recommend recording a small fragment first and listening to it immediately, you may want to adjust the recording volume or other settings. Next, record the entire tape.
The design of the cassette cover and stickers.
Find the highest quality image you are interested in and print it out. Cut it according to the size of your cassette case.
Fold the resulting sheet with the image according to the diagram for your cassette. The cover is ready.
Prepare the image for the sticker on the cassette. At the printing house, ask them to make you a sticker that fits your cassette.
The size of the cover for any case and stickers for all cassettes can be found and downloaded here https://boosty.to/kindaronin/posts/5bb9805e-cda3-4bd7-a0c2-fbf44411a48a?share=post_link
There are ready-made examples with all the files for high-quality music recording and ready-made options for making cassettes here https://boosty.to/kindaronin
Good luck creating your own audio cassette!