Open /billing → pick the client. You'll see two cards: "Log time" and "Log event".
Time: date, hours, description, hourly rate (defaults to the client's default rate). Mark non-billable to track effort without billing.
Events: for fixed-fee transactions — incorporations, board resolutions, AGM packs, filings, signatures. Pick the event kind, description, unit price and quantity.
What happens at invoicing: when you click "Generate invoice" the system picks up every un-invoiced time + event in the period, adds the monthly retainer once, and creates an invoice. Each line item is stamped with the invoice id so it won't get billed twice.
Tip: retainer is billed every invoice regardless — even if there's no time or events. So a client on a $500/mo retainer with a quiet month still gets a $500 invoice.