You'll need a pan that you can use both on the stove and in the oven (the one I use is 8 inches), some oil or butter, and the three main ingredients - three russet (i.e. baker) potatoes, some fresh rosemary, and a few cloves of garlic.
Preheat the oven to 400°F. Peel the potatoes and chop them into matchstick size pieces - don't worry about keeping them all the same size, because the idea is that they cook unevenly in the pan, so you get a mixture of crunchy and smooth.
Heat some oil or butter in the pan, and add the potatoes, along with the rosemary and peeled whole garlic cloves. Cook, stirring, for about 10 minutes, until the potatoes have softened and browned a bit.
Transfer the pan to the oven and let cook for about 20 minutes. Remove and smoosh the potatoes down in the pan - I just use my hand with an oven mitt on, but I suppose you could be more civilized and use a utensil like a spatula. (One version I tried used wax paper and a plate, but I found that the potatoes stuck to the paper, so I wouldn't recommend that method.)
Return the pan to the oven and cook for another 20 minutes. Remove and enjoy.