how to make a level editor in shooting game?

In 2d shooting games just like gradius,how to generate enemies by my will so i can really “design” a unique level not just randomly spwan them?

Normal enemies looks like been generated by specified time,and when you encounter a boss,the timer is going to stop so the enemies won’t appear…

But i just can’t figure out how to do it. To do this it seems I need to creat a level editor ,is the level editor a script?how to write it?

plz give me some advice or point me the direction.thankyou guys!
(I wondering if youtube have similar tutorial for this kind of game?)

There appear to be a number of 2d level editors in the asset store. Have you researched any of those?