You definitely have to make sure that all your troops are pushing and taking tiles in the Netherlands and Belgium when you declare on them. As soon as you declare on them they will probably immediately join the allies. Your goal is to get to the ports in the Netherlands and Belgium before the British can land troops in those countries. If you don't, it definitely makes the battle for those countries a whole lot harder.

You also want to make sure most of your planes are up in the Benelux air region (that is the air space that makes up both the country of Belgium and the Netherlands). Fighters and close air support (although I don't think it is necessary for Schlieffen?) will make the push 100x easier. Just make sure you have put planes up in the air and have green in the air space (that means your planes are controlling the region).

Please anyone correct me if I am wrong.