Make the script you want.
Add a setvar 0x(variable number) 0x1 into your script. Make sure that the player is FORCED to do the part that you add it into.
In A-Map, go to the map you want to put your level script onto.
Go to the Header view.
Under Map Script, remove all other old level scripts and press Add.
Under Script Type, select 02 Validates values, loads handler to...
Paste the script offset into the Script offset 2 box.
Put the number of the variable that will be set to 0x1 in your script under Flag. Leave the Value box alone.
Press Save map scripts.
Press Ctrl+H on your keyboard. The view should change to Pro-Header view.
In the Map options>Map script offset box, copy the offset past the dollar sign and paste it into the Offset box in XSE.
Decompile the script and...Blank! Not to worry! Press the button right next to the Decompile button, press the Decompile button itself and...voila! There's a script now!
There should be a minimum of three parts to the script: two things with pointers and your script that you made. In the second part, there should be a pointer to your script. Under it, there should be a #raw word 0x(random number). Replace the random number with 0. Compile it and test it out in the game.
Enjoy your level script!