The inputs are simple. You do up-B, and then during the startup frames (a very small time window), you press left (if you were facing to the right). This press to the left has to be done very fast. I won't discuss why this technique is useful, other people have done that. I just want to talk about how to do it.
Also, just to be clear, you can face right and hold up-left, and then hit B, and you will do the Dolphin Slash behind you and turn around. None of the information I've read is really clear about this, but I'm pretty sure reverse Dolphin Slash is different and requires doing it the hard way of up-B first and then press behind you second, separately.
At first I thought the problem is that my hands are slow. I'll just try it more and try to do it super fast, and then hopefully I'll get it. Well, I didn't get it. I went in Training Mode and tried in slow motion to make sure I was doing the inputs right. It worked. But at regular speed I was hopeless.
Then one day, I had a thought. You know what would save time? Don't push the dstick all the way up.
So I tried doing up-B, all by itself, without pressing the dstick all the way. And I found you only have to press it a tiny bit further than for up-tilt, but really not very far. Only a fraction of the way up is far enough.
The main reason I couldn't do it is because I was pressing the dstick all the way up, then pressing it to the side. And that takes too long. Maybe if you play in tournaments and you're really good, you could press it all the way up and still be fast enough. But I sure can't.
Well, once I had this insight, I was able to do reverse Dolphin Slash successfully about half the time in only 10 minutes of practice.
But I didn't just start doing it. I practiced an intermediate step that I think was a really good idea. If any guide had told me to practice it this way, it would have really helped me.
Press the dstick up half way. Hold it there. Now if you hit B, you will Dolphin Slash. Try it. So now instead of pressing up-B for dolphin slash, you start with half the work done, you just have to press B. Now do this: press B then, almost at the same time, press left (if facing right. press behind you).
When I just tried to hit up-B then left, it was so hard, I couldn't do it. But when I held up and then tried to hit B and left, it was so much easier, I could do it pretty much right away. It's not that hard to do one thing with your right thumb and one with your jump thumb, and do them very close together. Doing two things with your left thumb and something with your right, and coordinating the timing, that's hard. But only one thing with each thumb isn't too hard.
So practice that a bunch and you can learn the timing of when to hit left relative to when to hit B. Without a bunch of stress and failure. You can learn part of the technique by itself without having to be able to do the whole thing.
Once you're good at that, then practice the dstick motion without B. Press it up only a little of the way, definitely not all the way up, and then jam it left hard and quick. And practice it to the right also.
When that feels OK, then try another small step. Press up a little ways, pause for a split second, then press B and left. So it's like doing it with up already pressed, but instead of just holding up and not thinking about it, you do the up press only a moment early, so it isn't totally separate.
Once you can do that, then try to do the whole thing. And because of all the little steps you did, I bet you'll be able to do it sometimes. Not all the time, but sometimes. And once you can do something 5% of the time, then you have a good start and you just practice more and increase that percent. Whereas if you can't do it at all, it's hard to get started and you'll need some easier steps.
So you press up a little ways and B, and then hard left. It won't work every time. You'll get some neutral B (Shield Breaker) and some side B (Sword Dance) at first. But now you should have a good enough idea of how to do it that you can practice until you get it consistent. These little steps to work up to it will get your foot in the door and make the technique approachable.
Again, I'd like you to learn not just how to reverse Dolphin Slash, but also how to approach learning anything that's hard to get started with. This is both a specific example that will help Marth players, but also it's about the method of how to learn.
For part 1 at my blog, click here.
For all parts, and some people's helpful replies, see my thread at Smashboards.