I haven't installed one of those specific springs, but there is a simple generic way to install tension springs. First, set the mower height so that spring has minimum stretch. Then, you need a simple but special tool to do the next bit. Take a piece of fairly thick steel wire, and attach a T handle to one end. Then bend a simple hook in the other end. Put the hook through the hook in one end of the big spring, and attach the other end of the big spring to the mower. Ensure you are wearing safety glasses at this point. Now lower the T handle until the spring is horizontal and reaching in the required direction. You might be sitting down with your feet against the nearest wheel of the mower at this point, to keep it from rolling toward you. Draw on the T handle until the hook in the end of the spring is beside its attachment point on the mower. With a glove on your other hand, push the end of the spring across onto its attachment point. Reduce the tension on the T handle until the spring-end is sitting properly on its attachment, then wriggle the end of your special tool to unhook it from the spring. Put the special tool away, and drag it out every time you have to stretch a tension spring to install it.
