Q.how long after flowering for a watermelon to form
How long after flowering does it take for a watermelon to form and what will it look like?

As long as the flowers get pollinated, it shouldn't be too long until you begin to see the beginnings of your watermelon fruit, which will look like little green oblong balls at first. They will also need more watering during fruiting. If you have trouble with pollination, you can always hand pollinate the plants. Here is more information that should help you: https://www.gardeningknowhow.com/edible/fruits/watermelon/growing-watermelon.htm
https://www.gardeningknowhow.com/edible/fruits/watermelon/pick-a-watermelon.htm
https://www.gardeningknowhow.com/edible/fruits/watermelon/watering-watermelon.htm
https://www.gardeningknowhow.com/edible/fruits/melons/hand-pollinating-melons.htm