The class reference does not have examples of use, a page with examples for each class could be good to have (but not on reference), anyway...
Somewhere, like in _ready:
timer = Timer.new()
timer.connect("timeout",self,"_on_timer_timeout")
#timeout is what says in docs, in signals
#self is who respond to the callback
#_on_timer_timeout is the callback, can have any name
add_child(timer) #to process
timer.start() #to start
Then, on the connected node (self in this case, could be the parent or any other node in the scene)
func _on_timer_timeout():
your_timer_stuff()