Add each patient once on the Patients tab and their total billed, total paid and outstanding balance roll up on their own, with any balance flagged red. On the Bills tab, log each bill: date, patient (from a dropdown), service or item, amount billed, payer (Cash, M-Pesa, Insurance or NHIF) and the amount paid now; the balance on that bill is worked out for you. On the Payments tab, record any later payment against a balance, so a patient's account nets to the true amount still due.
The Summary tab totals the whole clinic: patients on file, bills logged, total billed, total collected, total outstanding and your collection rate. Yellow cells are the only ones you type into; every other cell is a locked formula, so the maths cannot break. Patient information is confidential, so keep the file on a device only your staff can reach, and if you use Google Sheets, share it with named people rather than a public link. It opens in Excel, WPS or Google Sheets, so you can keep it on the front desk or on a phone.