It depends on the context. You can say:
"..., that is to say,..." - This is good to use when you've said something that requires more explanation. You say the first part (the part that requires further explanation), then "that is to say", then the rest of the explanation.
Example: "A car is a vehicle, that is to say, it is a means of transporting people or things."
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"To say it a different way,..." - You can start your sentence with this. So if you say something, and the listener doesn't understand, you can continue your explanation with "To say it in a different way,..."
Example:
You: "That is a car.
Listener: "A what?"
You: "A car. To say it in a different way, its a vehicle that people use to travel."
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Those are a couple different ways. If you can give me some specific example situations, I can probably help you better.