Wheat is one of the Crops that can be found in NPC farming fields, especially west of Britain and east of Skara Brae. To harvest(or "reap") the wheat, double click the plant and a sheaf of wheat will appear on the ground that you may pick up. After harvesting, the wheat will be reduced to stubble but will slowly begin to grow again.
Wheat has five growth stages before reaching full maturity, but can be harvested by the 3rd stage when the seed stalks begin to appear at the top of the plant. There is no benefit to waiting longer than the 3rd stage to reap the wheat.
Four wheat sheaves can be processed by players using a Flour Mill to produce a Sack of Flour, which in turn is a common ingredient to produce food items with the Cooking skill. Note that when using a stacked pile of wheat, every time you target the mill you will use two units of your pile. After the mill has been loaded with four wheat sheaves, double-click the mill to begin the grinding process. After a few seconds of processing, a sack of flour will appear on the ground in front of the mill.