Charging the cost of inventory items to an activity is done using the inventory adjustments screen which is part of the Inventory Control System. For specific instruction on doing this, see the Inventory Control Manual. The transaction is simply a reduction of the inventory unit count and cost, and the subsequent transfer of the inventory item with cost to the activity.
As an example of the type of entry required using the Inventory Adjustments screen, let's assume that we have 6000 lbs. of steel scrap in our inventory. The inventory product code is steel. We want to move the 6000 lbs. of steel to the job we have created for the 20' pleasure boat. The data entry of this transaction using the Inventory Adjustments screen looks like this: