Everything is explained (hopefully well enough) in the video but here's the rundown anyway:
(1) Select 8-Ball
(2) set it to zero AI opponents, so you're playing in Practice.
(3) sink every ball expect the 8-ball (this just makes it easier).
(4) position the 8-ball immediately in front of any corner pocket. Be careful not to accidentally hit it in, but you can do this by aiming it slightly next to a pocket and not hitting it too hard.
(5) Whichever corner pocket the 8-ball is at, you want to put the cue ball in front of the other corner pocket on the same end of the table.
(6) remember that you can scratch on purpose to move the cue ball anywhere you want.
(7) store up a lot of power in your shot (by holding R2) and give it a good hit following the diamond pattern in the video.
(8) If you do this with any ball other than the 8-ball it doesn't seem to score enough points.
(9) If you make the 8-ball this way but scratch in the process, it may not count.
(10) I've done it this way twice, worked perfect both times, hopefully it'll work for you. If there's a simpler way I haven't seen a video for it or read about it, otherwise I wouldn't have posted this at all, so I hope it helps.