A Samosa can be sealed by joining the two edges of the dough. When you join the edges, it automatically forms a cone-shaped structure. In this structure, you have to fill the stuffing mixture and seal the bottom with the dough. For the sealing purpose, you should use water on the edges. Also, press the edges firmly so that they don't open in the oil. You can also use rice flour slurry to seal the edges nicely. For better understanding, I'd suggest you watch the tutorials available on YouTube.