Order matters, as it does on a real job: the cone washer goes on the tank floor before the fill valve drops in; the locknut threads up the shank under the tank before the supply line's nut goes on below it; the flapper and the handle go in before the chain joins them. Then set it: the water stops about 1″ below the top of the overflow tube, nuts go on by hand plus a quarter turn, and the chain gets about ½″ of slack.

    Grey: fill valve, flush valve, locknutRed: rubber (flapper, cone washer)Blue: refill tube, and water during the testGreen ring: a spot the part can go now (red: not yet)A gravity toilet tank: a fill valve with its cone washer, locknut and supply line; a flapper on the flush valve, lifted by a chain from the handle's trip arm; a refill tube clipped to the overflow. The fill valve's critical-level mark sits 1.2″ above the overflow tube, more than the 1″ that anti-siphon fill valves require. Follow the maker's instructions for your toilet and valve.