Assuming you've already got your French fries, the first step is to find some potatoes. Counterintuitively, the process of turning your French fries back into a potato will require potatoes, so you're going to want to head to the grocery store after you head to McDonald's. It's difficult to estimate the number of potatoes you need for a given quantity of French fries, but I'd say you're safe buying one medium-sized russet potato per 120g of French fries. If you don't have access to an accurate scale, you can simply ball up the French fries in your hand and estimate what size potato they would make and then buy a potato close to that size.