Put the flour and oil in a bowl, and add almost as much water as oil, and just gently stir it with a spoon.
200 g flour, 100 ml oil
Add the water bit by bit, it will look a little like a batter at first. If you add too much water, the pastry is easily rescued by adding a little more flour.
70 ml water
The flour will bind everything together and you will have a soft ball of pastry dough, ready to roll out.
Notes
NB oil needs to be measured by volume in ml. OR by weight, in grams. The weight in grams for vegetable oil is roughly 94.4% of the volume in mlYou need the same amount by volume of oil when making oil pastry as you would in grams. So if you are using 200g of flour, you would need 100ml of oil and around 100ml of water. Maybe a little less.