You need to build a track in Sandbox mode, make sure all the requirements are met, test the track and only then can you upload it.
From the Menu, go to Sandbox mode then hit New Level. Choose the area you want to build in. If you're new to the game, only The Populous Labs are available but that's enough for this achievement.
You will then want to go into Coaster Construction and build a track. It can be very simple, as long as it joins up. It could just be a simple oval loop, as long as you have a starting point (the piece it defaults to when you start up) and then some other basic parts to connect it all.
Once that is done, hit the View button (squares, where Back used to be) and select 'Set Level Type.' I reckon it's easiest with ScreamRider. Then name it (optional), change the Level Tags and set the Commendations. You can just leave it at the default settings for commendations, but if your track is very simple then increase the number of laps in Setup so that you can score the highest commendation (alternatively, drastically lower the requirements for commendations).
Next, go into Preview Camera Setup and place a few cameras. I don't know how many you need, I just spammed A to place several. Then repeat with Destruction Camera Setup.
If you go back a step in the menu (or open again with View), you can see the Level Checklist. If everything is checked, you will be able to press Test Level (in the menu when you hit View). If you can score the maximum commendation, the level is certified. Close the menu and now hit the Menu button (horizontal lines, used to be called Start) and hit Save Level. Choose a save slot, save it and then go to Exit Level.
Back out all the way to the Main Menu, and now go to Level Center (last option) and hit Share Levels. Find the one you just created, hit A, hit Yes and the level should get shared.
Shortly, the achievement unlocks.