One: find a Horse-chestnut tree that exhibits the tell-tale signs of leaf-miner damage on its leaves.
Two: pick a leaf and place it in a large sealable plastic bag.
Three: keep the bag in a cool,dry place.
Four: check the bag regularly for the emergence of adult Horse-chestnut Leaf-miner moth(s) and any parasitoids that may also be present.
I put a leaf in a bag on Saturday and had three adult moths today - result!