Its pretty easy I must say. While playing though, I experienced a crash:
############################################################################################
FATAL ERROR in
action number 1
of Draw Event
for object obj_control:
Unable to find any instance for object index '100034' name '<undefined>'
at gml_Object_obj_control_Draw_0
############################################################################################
--------------------------------------------------------------------------------------------
stack frame is
gml_Object_obj_control_Draw_0 (line -1)