I don't think that really answered his question. Most the time when you will dair someone off stage, it will be because of a ken combo. Unlike other combos, Marth cannot set this up with consistency. Meaning the opponent has to be in a bad position for them to get dair'd off stage most of the time. If you put them in a bad position you can punish them by taking a stock with the spike off stage.
If the opponent is in the air, you want to fair them first. Fair them close to them so you don't get a tipper fair (tipper fair has more knock back and will put them out of combo range)
Then you want to double jump up and or slightly backwards and push down on the c-stick to dair/spike. Watch people ken combo (YouTube) and you will see what I'm talking about. Look at some marth combo vids.
You can also utilt (with your back facing the ledge) and jump towards them (not facing them) and spike them with the backswing of dair against space animals and possibly other characters (situational). Most easily done against heavy/fast falling characters.
Those are 2 big dair spike set ups.
You can ken combo a grounded opponent but you have to shffl your fair and if they are grounded you want to get a tipper hit so they will pop up into the air. Then once again DJ up/backwards and dair.
You can also tipper fair a grounded opponent (shffl and land) to close fair opponent in the air to dair while still in the air
Just to reiterate; the spike hitbox on dair is along the tip of the swing. If you ever get the chance spiking with the backswing of dair is much much easier than getting the front half hitboxes.
Make sense?
Edit: I think I repeated myself about the grounded ken combo but I'm on my phone and I'm not fixing it lol.