1. Set up a line of water sources and have a line of piston set up 1 block below on the horizontal.
2. Surround the water on all sided but the piston one.
3. Have a line of repeaters running into the block below the water and a redstone line connecting them. Place 1 lever to turn them on and activate the pistons.
4. For 8 blocks long add your farmland
5. And the end have 1 block indentation and a waterstream leading into a hopper and chest.
6. (Optional) set up a sorting system to filter the wheat from the seeds.