Samosas are deep-fried filled pockets of filo pastry, often served as an appetiser in Indian cuisine. They are folded in a very specific traditional way, which hold them together very tightly. Samosas cannot be sealed in the same way as other pastry pockets, by pinching the edges together. There is insufficient excess dough to form a thick enough seal, and filo pastry is a solid sheet, rather than a thick paste. This prevents the two surfaces combining into one.
Make a diagonal cut at either end of a strip of filo, so the finished shape looks like the roof of a house, or a pyramid without the upper floors. It's easiest to do this when the filo is still in a block of many layers -- that way all your dough will be cut to the right shape.
Peel off the top two layers of filo. Using two layers will ensure that your samosas stay sealed while cooking. Lay the first sheet over your palm, so the short edge is nearest your fingertips. Fold one side over into the palm of your hand, so the pastry forms a point at your fingertips, and a small triangular flap overhangs the longer edge, by the heel of your hand. Paste some of the flour and water mixture on to the back of the rolled-over edge, and fold the other side over in the same way. You should now have a large triangular pocket, with two small triangular points poking over the open edge.
Pry the pocket open gently and spoon in your samosa filling. Try to fill the cavity completely -- when you're done, there shouldn't be any residual air inside. Fold the flaps of excess pastry over the open edge of the pocket, and "glue" them down to the side of the samosa, as if sealing an envelope.
Brush the counter down with cold water to help prevent the filo from sticking. Place a strip of filo on a counter. Have some more cold water nearby, as it is useful for wetting your fingers, preventing the sheets from sticking to them as you fold the samosa.
Place a spoonful of your filling mixture on the strip of filo. The pile of filling should be less than half the width of the strip, and positioned over to the left side. Take the right-hand corner nearest to you, and fold it over the pile, creating a triangular flap that covers the filling. Use wet fingers to press the edges down. Seal the edges of the filo pocket, and press out any excess air from around the filling.
Flip the pocket of filling over the edge furthest away from you. Work down the strip of pastry, squeezing out the air with wet fingers after each fold. Angle the pocket at 45 degrees each time, as if folding a flag. Tip it over on its short edge, then fold it to one side, so the long edge always stays in the centre of the strip. Keep going until you reach the end of the pastry strip. Seal it with a brushing of flour mixture.