You will need two crescent wrenches. You turn one while using the other to hold it steady. The flex connectors can have different ends (male on one, female on the other or two ends the same) so you need to make sure you have the right configuration before you start. Flex connectors don't require pipe dope because they are flares. Just be sure to use soap to test for leaks afterwards.
Anything that's related to a natural gas line I would hire a plumber to take care of so that there is no risk of gas leaks.
You will probably need to purchase a fitting that goes between the two lines that is actually made for this type of connection.
Yeah, like Toni, I would hire out for this as this is something I personally wouldn't want to mess up. I'm sure it seems easy but if it leaks, that's bad.