FROM THE FERRY:
From the Ferry turn right when you get to the junction and head along the road till you get to a traffic circle where you will take the second exit and head out to the north, the power station will be on your right and the car sales garage on your left, carry on along that road heading up the hill passing a petrol station on your left and a second hand car dealership, carry on for the next 8 miles till you get to Girlsta. When you find the turning for Girlsta turn in to the right and follow the road down to the stable on the corner with the green doors - follow the track alongside that comes down to Cheyne House (which is the brown house on the shoreline in the picture below).
When you leave the ferry and need to go to Tesco, turn left at the junction and head along till you get to a traffic circle take the second exit (basically go straight on) and follow the road which bends to the left and at that traffic circle turn right and head to the end of that road which takes you to Tesco - to get to the house just reverse these direction back to the ferry terminal and follow directions above.
FROM THE AIRPORT:
From the airport, follow the road (A970) and signs for Lerwick as I would imagine you would head to Tesco first, then follow the directions above to get to the house.
Please phone me if you have any problems or the directions are unclear and I will help you further.