Raising one
Sales → Invoices → New. Choose the customer, the date and the due date, then add lines with a description, quantity, price and tax.
Set payment terms on the customer record rather than per invoice, and every invoice you raise for them inherits the right due date.
What it posts for you
When you approve an invoice it writes its own ledger entry: debit Accounts Receivable, credit the income account on each line, and credit the tax account for any tax charged. You never post this by hand.
Quotes first, where there is one
Sales → Estimates is for a price you have offered but not yet earned. An estimate posts nothing to the ledger — nothing has happened financially until the customer accepts. When they do, convert the estimate rather than retyping it, so the two cannot drift apart.
Getting paid
Record money arriving under Sales → Receipts, and match it against the invoices it pays. That matching step is what clears the invoice from your AR Aging and stops you chasing someone who has already paid.
Record part payments as they arrive. The invoice stays open for the balance, which is what you want.
Correcting one
Never delete an invoice a customer has already received. Issue a credit note instead, so both documents exist and the history explains itself.
Recurring
For customers billed the same amount on a schedule, set up a template under Sales → Recurring and invoices are raised automatically.
See the sales chapter of the Kinxso Accounts manual for the full detail.