Part of what makes goose fat so great for roasting potatoes is its high smoke point, which allows the potatoes to cook at a high temperature without any risk of burning the fat.
Start by boiling the potatoes to soften them up, then preheat your goose fat before adding them to the roasting pan. Goose fat is semi-solid, so it needs to melt first.
Finally, flip the potatoes over a few times so they are fully coated before putting the roasting pan back in the oven and adding salt, pepper, herbs, garlic, and rosemary.