A good casette always has a good cover. You don't have to be an artist - it's better if you're not sometimes. You can cut up some thicker paper and doodle on it, stick a couple stickers, draw, cut up a cool picture or print something even - the possibilities are endless. Most importantly, have fun with it.

I will not go into detail on recording casettes now - you are on the internet after all, there is plenty of tutorials out there. But make sure to think of the two sides - depending on the length of the casette, you have half the minutes on side A and the other half on side B so you have to "split" the playlist into two say, 25m smaller playlists (if the casette is 50m total), if you don't want a song to cut off when the tape flips.

I personally believe the order of the songs is maybe the most important part in the whole ordeal. The formula goes as follows: start with one of the most ear-catching songs, but not the most ear-catching one - you need to get the listener's attention so use something they might be a little familiar with or one that you think they will like best. Then, take it up a notch - put on a song that is a little more energetic, to keep the interest. After that, we can slowly tone it down, but make sure not to go too overboard with the slower more "boring" songs, or else the listener might get bored. It is always good to change the tone slightly with each song. Finally, finish with a memorable one - people tend to remember endings. Of course, you can change up the formula however you like, but this is the most fail-proof one in my eyes.
 

Think about the mood - are we going for something easy-going and chill like soft rock or folk? Or maybe something more energetic like rock or punk? Is the playlist just one for you to listen to whenever you feel like it, or is it meant for someone - maybe you want to express something to them, or just give them some new song recommendations you like. It's best to write some ideas down and keep the ones you like best, that way you don't have to commit to anything yet.