I found that it helps to just think logically about the steps within each cycle.
First think of basic separation of duties–a different person to have access to the asset/resources, recording duties, authorization, and reconciliation of the records. There should also be documentation.
For purchases, individual department heads should authorize expenses, purchases. There should only be one person/dept separate from operational departments of the entity in charge of initiating the purchase and negotiating with vendors. Next, once a purchase is made, you should have the same identifier on the purchase order, invoice and eventual receiving document. The separate receiving department should get a document that does not include a count so the goods can be independently verified upon arrival. Then, AP examines the receiving report and compares with the PO and invoice before issuing payment approval. Mark all 3 documents as approved and record the transaction. Then Treasury issues payment.
For sales, you follow a similar approach. Instead of a centralized purchase department, you centralize credit approval. When payments arrive, you have independent dual counts of cash. Just like with expenses, you have regular reconciliation of bank accounts and aging schedules by persons not involved with recording the transactions. With sales, whatever is shipped should be recorded and vice a versa.
I may be skipping a detail or two, but just think about proper separation of duties or what you would need to be confident nothing shady exists.
AUD - 90
BEC - 79
FAR - 77
REG - 77
They don't trust JUST ANYBODY to count beans