GameMaker script newb question

Hey folks,
I'm making an endless runner with GameMaker.
Can anyone help me to get some basic spawning working?

I have an object at the right of the screen that emits one enemy object so far.
Eventually I will need to have several different types of enemies spawning from the right, out of view and scrolling past the screen.

I have a destroy object the height of the screen to the left so anything that touches it is dealt with.I know theres another way, I just haven't found it yet.
I'm not using xviews.Just a standard room with scrolling background/foreground and player stuck in the middle with a jump action.

Heres an event I created for the spawner object.

///spawner
var pop;
pop = 1;

if instance_exists(obj_enemy1) 
{
    pop = 0;
}
else if !instance_exists(obj_enemy1) 
{
    pop = 1;
    instance_create(x,y,obj_enemy1);
}

It creates one enemy ok.
I am trying now to get it to create more than one, preferably on a timer or depending on some other condition.

Does this code run evey frame/step?
If so, what did I miss?
I had thought this would reset the var/bool "pop" to 1/true at the end and rinse repeat the process of spawning if pop = 1

Maybe in the if statement I need a && pop = 1 ???



Also If anyone has a link to a site with a list of handy functions for coding GML I would appreciate it.
I keep finding bits and pieces for different types of games. Nothing so far that is teaching me the basics of GML.
Tried an example project for an endless runner and a good deal of it is a mix of code and drag and drop

Finally any other handy scripts or code I can use for this endless runner, would be very much appreciated.
I have till next week to finish it, so I am getting frustrated trying to get the basics of gamemaker in a hurry.
Unity seems easier in that all the code is done and used through the editor.
Gamemaker tutorials are a mix of drag and drop and code,...to me I find it a bit confusing, I would nearly rather it all code at this stage!