Inventory shows what is at or below its reorder level. That list is the honest starting point for an order.

Raise a purchase order

Order stock from a supplier, with the quantities and costs recorded so the delivery can be checked against them.

By the end of this the order exists with its lines, its costs and its expected date, and the delivery can be booked in against it.

You need a supplier on the system. Adding one takes a name.

Open Purchase Orders and start a new one
Inventory → Purchase Orders → New order. Pick the supplier first: it decides what you can order and at what price.
Add the lines
Search your own stock and add what you need. The cost comes from the item and can be changed on the line if this order is at a different price.
The cost is stored on the line as it is now. Changing the item's cost later does not rewrite what this order said.
Set the expected date
Used to flag an order that is late, so a delivery that never arrived does not sit unnoticed.
Save it, then send it
A draft can be edited freely. Sending it fixes what you asked for, which is what the delivery is later checked against.

Questions that come up

What should I order?: Inventory shows what is at or below its reorder level. That list is the honest starting point for an order.