To do this, first lay a green path. (using the 8x8 Tile Editor or copying existing green paths from the original overworld) Next, select the tile that will activate the event after you beat it, then click Overworld and Modify Level Tile Settings. Select the event you want to activate from the drop down menu. Next, go into Layer 2 Event Editing Mode, and then click on the icon with a picture of a white path. In this window, hit the down arrow until you get to page 7/8. Select the pieces of white path you need to form Mario's new path, and lay them down in the order you want them to appear. Finally, select the level tile that will appear when you activate the event, go to the same window as before, and select from the box which event you want the tile to be activated on. (It's the same one as before) Press OK, and you're done!
Few days old but meh, I'll give this a try. Start off by deleting every level off of Yoshi's Island (for example). To do this, click on the plain ghost house (Layer 1 16x16) at the top, click and drag over the mini map and hit delete. Next you want to get rid of any active events that are already present on this stage. Click on the glowing green hill to enter the event editor. You'll notice one path is highlighted in red, this indicates the event that's about to take place next (currently 0x0).
There are a total of 0x0 - 0x77 hexidecimal events in this game, so that includes A - F. To see these events, or navigate between them, you use the page up and page down buttons. If you press page up, you'll notice the events change and change, untill eventually the whole game is completed. Push page up enough times so all the events are revealed on Yoshi's Island, drag over and delete them. If you're using other areas, click on the glowing ghost house and do the same thing, as some events are programed with this editor. It's essentially the same thing, just, different, I don't know. But if they use it, delete them too as it makes it easier to work with. If you want to get rid of the sprites too, go for it.
Now you're pretty much ready to start fresh on this mini map. Click on the green hill and draw a little land to work with, make it a decent size, depending on how many levels you want to do. It's just as easy to make it simple for practice. First thing you should to in my opinion is plot the levels and work with the paths, as it doesn't always work out like you want to and you have to change things anyway. I personally find it easier to decorate after plotting the paths.
Okay, what we want to do now is click on the plain ghost house again to go into the level editor. Click on the blue switch palace to bring up your level tiles. To keep it easy, look for a yellow circle, before plotting it check "view mario paths" to make sure you're using a green one (meaning it's on land, blue means in water). Place it down somewhere sensible, somewhere where you have plenty of space to work with the paths. I'm not going to direct you through corners and stuff, you'll manage that as you pick it up, so for now choose another circle tile, except this time choose a translucent circle (meaning it's invisible untill activation). Place it down a couple of squares away (2x2) and go back to the level tiles, this time pick a straght piece of path that will join them together. You'll end up with a green circle, with a green line attatched to another green circle (with mario paths on of course).
Now we've set the paths, while short, it's time to add the event that takes place after completion of the first level. Before we actually draw them out we need to program the levels. Note you should now hold page down while in the event editor (glowing green hill) till you get back to event 0x0, just to be safe. Now click on the plain ghost house again and click on the level you want for the start point of the event. Go to the top and click on the little yellow circle, to edit the level settings. For this case, you would want to set the level to (for example) 105, as this is the first event, set the event to activate as event 0 and the direction you want him to move after completion, in the direction of your second level. Now, click okay and head over to the second level, this time set the level to (for example) 106, set the event to activate to event 1, the direction after completion and this time highlight to reveal this level after event 0.
What this means is that when you complete level one, event 0 will activate, event 0 will take place and level 2 will appear on event 0. Pretty simple really. You now just need to add the event tiles. Click on the glowing green hill and make sure you're on event 0x0 by pressing page down. Click on the yellow path button and it will bring up all the event tiles. Essentially what this is, is everything you see on the blue screen to the right. You navigate by pressing up and down which eventually go through all the objects. Because of this it's actually possible to create your own event tiles and paste them on the right next to the others, which is actually really handy. Like said before, just draw each tile in order one after other, in this case place the tiles that replace the standard layer following the path, so a white path maybe, then finally put down the white circle to surround the level dot. You should eventually have level one with blue squares highlighting the event tiles you just placed.
That's pretty much it. Before you save, move the Mario sprite over the first level save and go test your event. Use a fresh save file as the overworld isn't effected in already used files. If they're all used, just delete the .srm file from the rom folder. Oh, secret exits requre an event 1 higher that the level is programed for. You don't have to do that, but if you want an event after a secret exit then you need to make sure that any following levels get done after that. So say, level one has an event only activated by a secret exit, you would have to draw that event on event 1, 1 higher than event 0 which the level is programmed for.
Crap, that was long. I seriously thought I could type that in a couple of paragraphes. Oh well, hope it helps...