You can instance a node by using the preload() function to get a Node, which you can instance using the instance() function on the Node. Then you add it to the scene tree or a child node. Here's an example:
func spawnDust():
# Preload the dust scene (preload loads the resource at script parsing, rather than when the function is called)
var dustScene = preload("res://Resources/Objects/Dust.tscn")
# Create the dust node
var dust = dustScene.instance()
# Attach the dust node to the root node
get_viewport().get_child(0).add_child(dust)
# Move it to wherever you want
dust.translation = get_parent().translation
Here's the documentation page on it.
