Try digging a deep area just where the seed will be planted; add compost and/or other soil amendments, and keep the soil loose. They like good drainage. Once established, the sunflower (if given the right nutrition, light, water, etc.) might be able to outcompete the grass, as it can develop deep roots. When it is a seedling, though, carefully remove as much grass as possible surrounding the sunflower. The more grass and other weeds that you can remove, the stronger your sunflower will be.
You can try smothering the grass in that area with a thick area of newspapers (with wood mulch on top, or just wet the newspapers with a hose every so often so that they don't blow away); this normally takes months, though. If you need to plant your sunflowers right away, you might try digging a wide, deep hole as best you can for the flower, adding the soil amendments and loosening the soil, then planting the seed, filling in the hole, and then covering the area with the thick newspapers, cutting out a small circle just where the seedling will emerge. It should cut down on any grass or weeds in that area.
Adding soil on top of the grass can help, but the grass will eventually poke through it, and it helps to get the soil nutrients deeper so that the sunflower's roots can more easily take advantage of them. But, gardeners have had great success growing sunflowers in raised beds, so as long as the added soil on top of the lawn is at least 6 inches or so, you will probably be fine. Just keep up with any weeds or grass that poke through.
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