Mower deck How can I remove slack from deck belt

My elderly neighbor has a Craftsman lawn tractor model # 917.28856 with 24 HP engine and 42" deck. The mower deck belt seems to have too much slack in it and I have been looking at his owner's manual for a solution. the belt is installed correctly and the tension spring is installed correctly on the idler pulley.
I really don't see any instructions as to how to adjust the belt. Am I overlooking something? Any advice will be appreciated.

Thanks in advance.
Red Cox

Mower deck How can I remove slack from deck belt