sit down to (do something)
To begin or prepare to do something while sitting. After putting it off all day, I finally sat down to do my taxes. I think it's important that we all sit down to eat together in the evening.
sit down to something
to sit down at a table to do something, such as eat a meal or attend to some business. I look forward to going home and sitting down to a quiet supper. Ted looked forward to sitting down to a big Thanksgiving dinner.