To do it properly you really do need a temperature probe - they cost about 12 quid.
Cut potatoes into 2cm thick chips
Heat water in a saucepan to just under boiling and make a brine by adding 15g salt, 10g sugar and a pinch of baking soda per litre of water that you're using.
Put the chips in the saucepan and cook at just under boiling - 90C - for about 25 mins.
Remove from the saucepan and place on a wire rack not touching so that they can cool and dry off - you can speed this up by putting them in the fridge.
Meanwhile, heat up the oil to 130C - this is where the temp. probe comes in handy. You want to use quite a lot of oil so that the temperature doesn't go down too much when you put the chips in.
Deep fry the chips for 8 minutes and then remove and put back on the wire rack to cool and dry.
When cool put them in the freezer and from then on you can treat them just like frozen chips.
When you're ready to make the chips, heat the oil to 190C and deep fry the chips for 3 minutes.
An alternative to this that you might want to try is to buy something like McCain oven chips but deep fry them instead of doing them in the oven. This does actually produce some quite good results as well - certainly much better doing them in the oven