Cut the board to the required width and length. There are several methods used for cutting cement board:
Circular saws. Special saw blades are available for cutting cement board, but carbide toothed circular saw blades will give satisfactory cuts if the amount of cutting is not excessive. Avoid breathing the dust produced by sawing, either by using a fan or wearing a NIOSH approved respirator.
Pneumatic or electric sheers can be used for thin boards, but are expensive and usually only practical for a professional installer.
Scoring and snapping. Using a straightedge and scoring the board with a sharp utility knife, then bending it (similar to cutting gypsum wallboard products) is a technique that can be used––it produces very little dust, but is less precise and doesn't give a smooth, flat cut.
Mark the location of any electrical devices or other items which will penetrate the sheet and cut holes for them. Be very careful if large holes occur, or if holes are very close to the edge of the board, as this can cause breakage if carelessly handled in installation.
Pre-drill holes for fasteners if desired. Screws can be driven through the board without pilot holes, however, if a certain spacing is required, laying out the screw locations and drilling pilot holes will make meeting this requirement simpler, and will make starting the screws easier.
Lift the sheet into place. Sheets must be supported carefully, or they will break, and because of the weight of the material, at least two people should be engaged in this step. If the installers are working from a ladder or scaffold, a ground person may use a piece of lumber to help hold the panel up while the installers position the piece and tack it in place.
Drive all fasteners into the panel, making sure to sink them flush with the surface or slightly below it, but be aware it is possible to drive screws too deep, causing them to fail.
Fit each subsequent sheet or panel into place until the area your are working in is complete. Install trim and caulk joints as needed, clean up surfaces where blemishes or spoiling from screw penetrations has occurred.
Prime and paint the soffit or ceiling according to manufacturer's recommendations.
Selected from: https://www.wikihow.com/Install-Cement-Board-Soffit-Panels