How do I use DAP correctly at sowing instead of throwing it on top?

Your neighbour is largely right. DAP supplies phosphorus, and phosphorus does not move in the soil the way nitrogen does. If you scatter DAP on the surface, the roots may never reach much of it. The correct method is to place DAP in the soil at sowing, in the seed furrow or a band a little below and to the side of the seed, using a seed-cum-fertiliser drill where possible. Banding near the root zone gives far better use of the phosphorus than surface broadcasting. One caution: do not let concentrated DAP sit in direct contact pressed against the seed in large amounts, as it can affect germination; a small gap or banding avoids this. DAP also carries some nitrogen, so it gives the crop a starter dose at sowing, but it does not meet the full season nitrogen need, so plan your urea top dressing separately. For the per acre quantity, go by your Soil Health Card or soil test and your KVK recommendation for that crop and soil, and do not over-apply; if your soil test already shows phosphorus is high, you may even be able to reduce DAP.