take the belt back off the left ( non discharge side pulley )
Reattach the tension spring
Pull the entire tension arm to stretch the spring
Slide the belt over the left side spindle pulley
It is much easier done this way
Use a hook similar to what Star has shown and slip it over the tension arm
You can also use a ratchet tie down, some rope or double nut the idler pulley bolt and put a long wrench on the nut to get the leverage you need to stretch the spring