The Yeti piece starts in the summit station of the scenario you're playing.
The Yeti moves down and up the mountain when the weather is processed - it moves from station to station, like the Surveyor.
If the Yeti moves into the same space as one or more Surveyors it will stop and each Surveyor's Player - in turn order - must pay 1 coal to supply or move one Station down the mountain.
If a Surveyor moves into the same space as the Yeti, the Yeti will move one Station up the mountain unless the Surveyor's Player pays 1 coal.
When a Player pays 1 coal, the Yeti will perform an Excavate or Build action immediately in that Station as if its was the Player's Labourer - if Building, the Player must pay the appropriate Building cost.