Sit yourself down
Definition 1 of 2
Pro Tip 1/3Casual Setting
Used among friends or in relaxed environments, not for formal situations. 
Hey, sit yourself down and chill for a minute.
Pro Tip 2/3Emotional Prep
Signals preparing for something emotionally significant, often used before sharing news. 
Sit yourself down, I have something important to tell you.
Pro Tip 3/3Long Duration
Implies that what's coming is going to be lengthy, like a story or discussion. 
Sit yourself down, this is going to take a while.