First, you need to know how much you're making. So take a pot (one with a lid available), and put an amount of potatoes in it to help you visualize the amount you're gonna get from them

Peel potatoes then wash them

Cut them up into pieces, smaller pieces cook faster, try not to make them too small. Then put them into the pot

Fill the pot with water and (using the lid) pour out the water to give them a final wash

Pour water again into the pot just so that the potatoes are covered with water (it's okay if they slightly poke out

Note: not all of the salt will deep into the potatoes, a lot of it will stay in water, but the more you add the more will enter the potatoes

Cook the potatoes on medium to high heat, and put the lid on

When it seems to almost boil over, move the lid a bit so there is a gap, you can also lower the heat a tiny bit but make sure that you have on medium at the very least

To check if they're cooked you can do two things: 1) use a knife and try to stab one potato, if it goes in nicely then atleast he outside is cooked 2) lift the knife above the water with the potatoe still impaled, if it slides off then the middle is also mostly cooked and may be ready

Once cooked, use the lid like in step 4 to pour out the water

Add a fair amount of butter (I used a quarter of a 250g butter brick)

Add some milk to your potatoes (I used about half a cup) and continue mashih until it's mashed well