Firstly, pick as many of the leaves off the coriander plant as you can, put in a bowl and set to one side. Cut off all the stems from the plant (and as much of the root as you can without adding loads of soil!) Place them into a pestle and mortar and grind down into a paste. You may need to chop it a little after to stop some slightly stringy bits from clumping together. You want to end up with about 2tbsp worth.
Alternatively – if you can get coriander root, use 2tbsp/20g of this instead. Coriander root has a very intense flavour but it is incredibly difficult to get a hold of over here. This was the ingredient used in the original Thai recipe. Note, you will still need some coriander to put on the top of the finished dish to add a decorative flourish & more flavour.