Previous climbers had cut
steps in the ice.
R: The ice the climbers got had cut steps.
Steps ... ... ... by previous climbers.
We use this room only on special occasions.
R: On special occasions we used this room.
This room ... ... ...
They Are carrying heavy suitcases.
R:Heavy suitcases are being carried by them. (You did this one almost perfectly.)
Someone will serve refreshments.
R: refreshments had been served. (Keep will: Refreshments will be served.)
You've got the right idea in the last two. You put the direct object (underlined) of the active sentence as the subject (at the beginning) of the passive sentence. But note that your placement of words is not correct in the first two. You didn't find the direct object correctly. It's not always the last words in the sentence. The underlined items are direct objects in the active sentence, so you put them first in the passive sentence. Try the first two again. OK?
CJ