Plain English is perfect — write it the way you'd explain it to a new member of staff.
e.g. year-end accounts and VAT for 80 clients, payroll for 30
VAT quarters, year-ends, payroll runs, confirmation statements
Fixed monthly fees, hourly, per-job — and your payment terms
Engagement letters, records in, accounts out — and where they live now
The workarounds, the spreadsheet on the side — that's the part we tailor around
Monthly fees, per-use charges, commission — whatever you pay