I used an image of the helmet from
Amazon for reference. I used Mirror Modifier to start work, so only need to make one side of the helmet. I started making it by a series of extruding, scaling, and placing vertices on the rough boundary of the helmet. Follow it by using Solidify Modifier to add thickness to the model. Now I use the model head to transform the helmet from a 2D plane into the third dimension. And continue to refine the mesh by subdividing continuously and working on the highest viewport settings.