Well, this sounds pretty basic, since it only is one of the several tipes of stairs you find in the SMB map.
There's a lot of ways of finishing a level, and only explaning one of them doesn't feel correct (to me at least), so i'll try to give a better explanation about the topic:Finishing a level (Guide)
There's a lot of ways of finishing a level (i just said this but saying it again doesn't hurt anybody): you can go the classic way and do the stairs of SMB, and make a lot of interesting and entertaining variations; or, you can do like in NSMB series and make obstables to help the player reach the map (this has a lot more possibilities); OR if you are making a story map or just feel like it, you can finish the level using an animation (only possible using Mari0 SE).First step: Make a decent level.
You can't make a mappack with only level endings. So go ahead and make a decent level with platforms or puzzles or both or whatever that you think the public will like.
Okay, now to the actual level goal:1. The "Classic":
This is the easieat one, but that doesn't mean is any lest funny to make or to play.
You start following the basic design and then add pits or pipes or make it shorter or make it larger, etc. (i.e. The "stairs" in the level 8-3 of SMB is made by only 5 blocks) But remember that it has to allow the player to reach the top of the flagpole.
Then the space between it and the flagpole will depend of how the heigh you decided to give to your stairs, as long as it's possible to reach the top, it's alright.
Extra: The castle after the flagpole doesn't have to be like the basic one, you can give it different shapes; and it doesn't have to be a castle, it can be a house, a cave, etc.2. The "New":
Sometimes, in the NSMB series, there's not stairs, instead you have to run and jump using different obstacles to reach the top of the flagpole, you can do that to in Mari0, and even you can directly send the player to the top. Since Mari0 has a lot of entities that could help the player reach the top, i really suggest using them.
Extra: Or you can just put the flagpole there, like in the Portal mappack (which is less funny, but at least it lets you to beat the level).3. The "Other":
For those who hasn't played it, i really suggest you playing Hugo's mappack Pop a portal
(if it's still compatible with Hugo's mod
In that mappack Hugo used an interesting transition from level to level, you get to the elevator and it loads the next level, without flagpoles or castles.
You can do that in Mari0 SE, making scenes and dialogue too, or imiatting that sinken' ship level in SMW (which had a green orb only used once in the game).
So, there you have it, i didn't give exact explanations, you have to do all the specific processes, since it's your map. So, go wild and go make some cool levels with cool endings with cool stuff. Good luck :)