You can go off the stage and if you see them go for the ledge you can stall with DB and your second jump and then when their invincibility runs out you can spike them from the ledge with your DS.
If they're in a worse situation, say without a second jump left then you just need to think about their leftover recovery options and time it right. Remember DS is pretty much one of the fastest moves in the game off stage (its one frame slower then fair coming out) and it makes you f***ing invincible during that lag anyways. So just be confidant in marth's off stage game, because it's really good. You are floaty, with plenty of time to stall, yet DS is practically instantaneous. Just be calm, and think about your options, you can always DS back onto the stage if you feel you need to abort for what ever reason. Your opponent should usually be the one who's scared off stage, not you.
Otherwise its just more of a situational thing and you have to be paying attention to see if your opponent gets themselves in a bad situation. There are usually better options for edge guarding or recovering and I would recommend those because they're safer, DS stage spike is really awesome but its not something you should rely on. Marth has much better off stage option than that usually.
Things to remember though:
-low percent Dair spike -> DS stage spike is very easy against a lot of characters because they will have to try and go straight for the stage to not die and will be very easy to read this way, just work on timing.
-first hit of DB then DS works very well, and if they airdodge that's fine because now you're on the ledge with your invincibility frames and your second jump back looking for a dair spike instead, or some other edge guard.
-don't be stupid and try this on somebody like MK or ROB because that just won't end well