With old potatoes, if you want decent crispy roasties, here are Two Additional Steps
1. After peeling, “turn“ each potato piece. Meaning, take all the sharp edges off the spud with knife or peeler, like you would take the arriss off, say, a window ledge timber before undercoating, so the paint sticks. This way (and chefs waste a lot of spud in turning them all to the same shape and size - don’t do that!) you don’t get burnt edges, but wholly crispy outers.
2. After par-boiling and cooling, “scruffle” each potato (this word I made up). Meaning, take a fork and scrape the entire surface of the floury delight until it is thoroughly scruffled. This way, you will get a fluffier and yet deeper crispiness. The skins will shatter between your teeth, and your dinner will be the envy of the neighbourhood.
Another step, if you so choose. Once turned and scruffled, after the par boiling and the cooling, place the fluffy delights on a baking tray and into the freezer (or the blast chiller - don’t we all? - or into your liquid nitrogen bath) and let them chill right down before all the other steps you’ve been given. The next one should be swirling on hot goose, duck or rendered lamb or beef fat before the roasting begins.
Gorgeous.
Please, please, please do not buy ready-made. They are insanely tasteless, hopelessly filled with bad fats and ridiculously expensive.