Question: how start atx power supply?
Answer: Step 1: Search! Just look after the green wire in the cable tree from the PSU. Follow it to the 20 or 24pin ATX plug. ... 
Step 2: Connect! Now it's time for the wire. Bend the wire and close up the little green plug from the big ATX plug to any other BLACK plug. ... 
Step 3: Extra Options. Add a switch instead the wire!