Mix and rub oil into the flour till you can make firm balls with your hands.
Add a bit more oil, if necessary.
Add water, a little at a time, to make a very stiff dough.
It should be so stiff that you should not require any extra flour for dusting while rolling.
Knead for 5-7 minutes, roll into a large ball.
Cover with a damp cloth and set aside while you make your filling.
Add cumin seeds, ginger and green chilies.
Give it 4-5 twirls, add potatoes and everything else.
Sauté for two minutes.
In a cup, make a thick paste with 1 tblsp of flour and 1 tblsp of water.
This is the 'glue' with which you will seal the samosas.
When all the above is organized, make walnut-size balls with the dough and roll each out to make circles of about 6" in diameter.
With a sharp knife, halve each circle to make 'D' shapes.
Put a little paste on half the length of the cut side and overlap it with the other half to form a cone.
Lightly press the glued edges to seal.
Fill the cone 2/3rds with the mixture.
Again, apply paste on the insides of the open edges at the top.
Press to seal the glued edges.
Your triangular shaped samosa is ready to be fried!
Make all the samosas--remember to cover them with a well-squeezed damp cloth.
When all are done, heat oil, 3 inches deep, in a wok/fry pan.
Slide each samosa in--do it in batches of 6 or 8.
Once you have a batch in the oil, reduce heat and fry on medium-low till they are golden brown and nice and crisp.
Serve hot with ketchup or coriander/mint chutney.