1.Choose the product you wish to install. Soffit and ceiling cement board products are available from a number of manufacturers and have a variety of surface finishes. You can research these at building product retailers, lumber and building supply stores, and larger home improvement warehouses. Here are some things to consider:
Dimension. Sheets can be in various sizes, from 12 inch (30cm) widths to 48 inch (121cm), and in lengths up to twelve feet (3.6 meters). Thicknesses are available from 1/4 inch to 1/2 inch (6.5mm to 1.3cm).
Surface. The surface of these products are available in rough-sawn lumber finish, faux stucco, and smooth.
Perforated or non-perforated. For ventilated soffit applications, the cement board product is available in perforated sheets, for non-ventilated, solid sheets are available.
Determine the framing requirements for the material you choose. Cement board requires support at a maximum of two foot (60cm) spacing, and additional blocking may be required at sheet edges to insure the surfaces are flush after installation. The best results are achieved when blocking is installed at all edge and end joints. Because of the weight of this material, joists or other framing members must be capable of supporting the total weight after installation.
Choose appropriate fasteners. For ceiling and soffit installation, exterior grade, paintable screws with countersink heads are required by some manufacturers. These should be able to penetrate the framing member a minimum of four times the thickness of the panel being installed. If you choose to use nails, hot dipped galvanized or stainless steel siding nails are recommended.
Gather your supplies, including the fasteners, caulking, ladders or scaffolding, screw-guns, drills, cement board, and hand tools. Install all framing members and roofing components to assure the assembly will not be exposed to weather until the installation is complete.
Place the cement board sheets on a flat surface with sufficient supports to keep them from bowing, and keep them dry before installation.
Measure the width and length of a space you will install a sheet of cement board on. Allow enough space so that positioning the board can be accomplished without damaging the edges, but make sure any exposed joints will have an acceptable appearance. Trim or batten strips may be used if joints are not tight enough to offer an acceptable appearance.
Selected from: https://www.wikihow.com/Install-Cement-Board-Soffit-Panels