The main goal of the day is to make Decisions. The decisions are required for tomorrow's prototype.
To make those decisions while avoiding the parole of group decision making, there are multiple activities to aid you.
Start with The sticky decision. Start by looking over the sketches from yesterday, think about the parts you like and the parts that you dislike. This is the part where you use those handy dot stickers.
Next activity is the structured critique. This when the group members say their mind about what parts should make it to the prototype.
Now it is the task of the decider to make the call. The decider has to take into calculation the smaller dot stickers on the parts of sketches that people liked. However, ultimately he has two large dot stickers that will decide the next day's prototype.
At the end of this process you may end up with more than one great idea for the prototype. This is no problem as there is an extra step to handle this very problem called the Rumble.
You will have to start by creating storyboards of the plans that you choose. The storyboards will act like a blueprint for the prototypes. Remember to take the existing sketches and use the whiteboards to re-stick them and draw the storyboard. Try to use everything that already exists and not come up with new components or ideas on the go, if by day three no one came up with that idea, there is a reason for it.
If you had to use Rumble due to multiple good ideas, now is the time to compare the finished storyboards and have the decider choose one. The prototype for tomorrow.