1- OPEN THE GTA3.IMG FILE WITH THE IMG TOOL PROGRAM
2- Delete the old mansion3a.dff file and add the new one, the modded one
then add the saveroom.dff and saveroom.txd files.
Delete the old starfish.col file again and add the new one there.
3-Go to the data/maps/starisl folder, delete the old folder and add the new modded folder or change the folder.