Do these once, before the first bill of materials.
#Work centres
A work centre is a machine, cell or bench that performs work, with the hourly rates that make its time cost money. Manufacturing → Work Centres → New work centre: Name *, Code, Capacity, Labour rate / hour, Machine rate / hour, Overhead rate / hour (the Total / hour is calculated), Fixed asset (the asset this centre is, when it is on the register — its maintenance cost and history then live on the asset, section 11.11), Description and Active.
#Operations
An operation is a named step — Cut, Weld, Assemble, Pack. Manufacturing → Operations → New operation: Name *, Code, Default time (minutes), the default Work centre, Description and Active.
#Routings
A routing is an ordered sequence of operations. Manufacturing → Routings → New routing: Name *, Code, Description, Active and the Operations list — for each step pick the operation, the work centre it runs at and the time. Routings are snapshotted onto each bill of materials that uses them, so changing a routing later does not silently change the cost of existing products.