This episode is a must watch, in my opinion. If I had to say when you should watch it, I'd say that it's probably best to watch it right after Aomine meets Kuroko in the Teiko arc during the third season of the anime. Essentially, rather than seeing the entirety of Kuroko's time at Teiko, this is a narrowed down summary of that time that just focuses on Kuroko and Aomine. It'll take you from the time they began their friendship to the final moments of it before they grew apart and went on to different high schools. I think showing the growth of Aomine andKuroko's friendship was beautifully done. You'll see Aomine supporting Kuroko a lot and encouraging him to continue basketball, which is basically the opposite of what happens in the current timeline. Also, there are quite a few similarities between Kuroko and Aomine first meeting and Kuroko and Kagami first meeting, which I found simultaneously entertaining and sad because that made me think that Kuroko took to Kagami so quickly because he saw Aomine in him and longed for a friendship like he used to have with Aomine. An example of one of these similarities is that, in the beginning of Kuroko playing on the Teiko team, Aomine wore the number 10 jersey, which is what Kagami wears at Seirin.

It doesn't end happy, so don't expect that, but you will get to see Aomine and Kuroko having fun together--and that includes their first fist bump, which is iconic. As I said before, you really should watch this if you're a fan of the anime. It provides a bit more perspective on Aomine's character that will help you enjoy his character more.