Clamp one end of the sping in a vise. Pull down on the other end with a bar untill it's stretched real good. Then stick washers between the coils to keep it extended, and wrap duct tape around it to keep the washers in. Take it out of the vise, and install it on the car. Remove the duct tape, and close the hood enough to release the washers. Remove the washers, and use them to do the other side.

Or you can buy the Kent Moore tool for hood springs, if they still sell it, mine is about 40 years old, I got it new. The washer trick will work though.