To make a scenario you will need  the saved game modifier.
You also need Loopy Landscapes.

Make a park the park needs to look how you want your scenario too look.
Save the park.

Open the Saved Game Modifier and open the park you saved.

Change the settings to what you want.

Change the scenario settings and choose a slot for it to be place in. (eg. 101)

Save it: You must save it in the scenario folder of the RollerCoaster Tycoon directory. Make sure the 
"save type as" drop down menu says "RCT Scenario (*.SC4)" call it what u like and click save.

Open RollerCoaster Tycoon. Go too start new game, their should be a new section called
competitions. Click on it and your scenario should be in their.

If you need further help then please email me.