you will need an open (no lid) deep fryer , if you only have the kind with filters and lids , just don't close them, the steam has to be able to leave very fast else you loose crunchiness.
Peel your potatoes , and cut them lengthwise into discs of about 1/4 inch thick (they can be a bit thicker if you like that) . Then slice the discs lengthwise into fries of the same thickness.
Make sure your sink is clean , and wash your fries in cold water . Drain the water , and wash them again in new clean water. Repeat this a few times until no starch comes off the potatoes and the water stays clear. Dry your fries well , so there is no more moisture on their sides.
First cooking . Put the thermostat for your oil to approximately 320°F or 160°C , not higher . Put a small to medium amount of fries in the cooking basket ( you will have to do this multiple times , if you put in too much fries at once you will loose your temperature and your fries will turn out horrible) . Make sure all fries are under by shaking the basket . During cooking you can shake the basket a few times so you know they're not sticking together. Important: To check if the first cooking is done lift your basket out and pinch a fry if you can pinch all the way through they are done (if you do it fast you will certainly not burn your fingers ), they should still look pale and white , stay with your cooking for this , timing is important.(repeat until they are all done , and let your fat heat up to the appropriate temperature each time).
Take out the fries , shake them and put them in a big bowl to cool off , once they are cooled to room temperature they are ok to start the second cooking . you can keep these at room temperature for hours until you are ready to serve the meal , in fact leaving them a bit will only make them better , just put a towel over the bowl , no need to put them in the fridge(where condensationwould coat them).
Second cooking. Put the thermostat of your oil at approximately 370°F-374°F or 185°C-190°C , this is usually the highest setting , or just below that . Put a _small_ amount of the precooked fries in your basket . fry them while shaking the basket in the oil a few times during cooking , you can even take the basket out a few times to throw them in the air (not really needed , but supposed to be good tor making bubbles appear on them :) . They are ready when they are a rich golden brown in color . I put them in a deep plate with a paper towel at the bottom , and use a paper towel to quickly rub the excess fat off them before i serve(paper towel on top hand on paper towel , make a circular movement , take out towel). add a bit of fine salt , and serve. best with a T bone steak or sirloin *grinn* but as you know , fries go with just about anything :).
A bit of added history : The French will be the first to say that "les frites" come from Belgium. And it being true that Belgians have made and perfected the stuff for years and years, the origin of deep frying potatoes in oil comes from a monastery in Spain.
Smakelijk (bon appétit).
Recently I found out that Bintje potatoes have become commonly available in the United States . These are without a doubt the best potatoes to use.