Make sure you have someone to help you hold the sheet while you cut it. It's important to wear safety gear while you handle and cut the underlay, so put on your gloves and dust mask. Use the fibre cement sheet cutter to cut across the line you have drawn.
Lay the piece of cut underlay on the floor, and make it flush to the doorway. Then mark 3mm in from the edge of the doorway. Draw an arrow onto the underlay to show the direction you need to cut, either left or right of the doorway.Measure the width of the doorway and halve it. Then mark that distance out from the 3mm mark on your underlay. Use your level to draw a straight line from this mark to the top of the sheet. Then cut the underlay and lay it flush against the doorway.
Measure the distance from one wall to another and cut your underlay to size. Make sure you leave a 3mm gap between the walls, so you can silicone seal it later. Remember you don't want to install underlay where a shower or toilet may be, so cut the underlay to suit. After you've cut all of your underlay to size, lay it on the floor.
Now you're ready to nail the underlay into the floor. There are places marked on the underlay to make it easy for you to hammer in the nails. Make sure you space the nails out evenly apart along the edges of your underlay.
Use your caulking gun to apply a wet and dry silicone sealant to the gaps between the walls and the underlay. This will make sure that no moisture rises up between the underlay.