A new sheet of polycarbonate has a protective paper layer. Leave this on while cutting and drilling. This visor is 30cm x 15cm. It is held on with aluminium brackets, nuts, bolts and felt washers and has lasted over 10 years. Use a slow, medium saw to cut the plastic. A bandsaw works well.
Remove the protective paper layer, place the shape on a piece of cardboard and heat to 180C in an oven. Wear thick gloves, remove the cardboard and hold in a curve for several minutes. It would be a good idea to practise bending the cardboard first.
Mark, drill the helmet and attach the visor.
If your clear plastic does not have a protective layer then apply a length of masking tape. A fine toothed blade on a bench saw can also be used with care.