Make sure the blade engagement handle is in the "disengaged" position. If you are working on the two deck belts themself, you may need to remove a spring, get the slack needed and put the belt on.
Trick to remove and install springs, first use eye protection.
Then use a piece of small pull starter rope cord and run a loop
Around the hook on the spring, you can pull the spring open further
This way for putting on, or removing the spring.