You can buy a potato ricer or vegetable mouli from large supermarkets, kitchenware stores and online. They’re very handy for mashing potatoes.
You must work with hot potatoes. The glutens in the flour need to react with the potatoes when hot.
Cut the potatoes evenly, as just one undercooked potato will give you lumpy gnocchi.
Gnocchi are best made and eaten on the same day.