Why this matters more than it looks
Sales are recorded automatically because they go through invoices. Expenses only exist if somebody enters them. A business that records every sale and no expenses has a revenue figure it mistakes for profit.
Supplier bills
Purchases → Bills → New records a supplier invoice. On approval it posts: debit the expense, COGS or asset account on each line, debit any reclaimable tax, and credit Accounts Payable.
Enter bills when they arrive, not when you pay them. That is what makes your AP Aging show what you actually owe, and what puts costs in the month they belong to.
Paying them
Purchases → Payments records money leaving, matched against the bills it settles: debit Accounts Payable, credit Bank.
Attaching the paperwork
Attach the supplier invoice to the transaction as you go. Hunting for it a year later, when somebody queries a figure, is the expensive version of the same task — Documents keeps it with the transaction that references it.
Expense claims
Where staff spend their own money, expense claims are recorded against the employee and reimbursed, so the cost reaches the right account and the reimbursement is tracked rather than remembered.
Returns and over-billing
Purchases → Debit Notes is the supplier-side equivalent of a credit note — for goods returned or an amount over-billed.
The purchases chapter of the Kinxso Accounts manual covers the full cycle.