Cook the potatoes in the oven at 200C for an hour - 2 hours, previously pinching them with a fork to help them release vapour. You can take them out once you pinch them with a fork or knife and it goes in easily.
Cut them in half and scoop out the inside of the potato with a spoon. Pass it through a sieve (once) or potato press (twice) while still hot. If they become cold before you can get to it, reheat them in the microwave. It will make it 100 times easier to press.
Mix in the egg, then add the flour and salt. The amount of flour in the recipe is the minimum amount that you need for the amount of potato, but add more if it's still sticking to your fingers.
Roll it out into a snake and then cut the size of the gnocchi that you want.
To roll them into gnocchi shape, roll them into a ball, then into a bit of flour, and then roll it on top of a gnocchi board or a fork.
At this point you can freeze them. If you want to do that, lay them on top of a container or a plate with cling film dusted with flour. Once they are frozen solid you can place them in a container altogether as they won't stick to each other now. This is the only way to store them because if not they become sticky.
Cook them in boiling water and remove them as they come floating. Put them on top of the sauce once done to reheat.