4. Click the helmet and press "alt + x" in the same tame so that you get wireframe, and than "freeze" it with the right mouse button. With the same technikue that we got the helmet make a spline like in next picture. This will help us make the face opening on it, add the extrude modifier.



5. Increase the amount of extrude and switch to perspective view, now pull the new object so that it goes through the helmet as in picture, right click on the viewport to unfreeze all.



6. Click on the helmet and than create -> compound -> boolean, than on option "pick operand B" and click on new object. You shoud get a whole in the helmet as in picture.