Assuming you are in England, your au-pair would need to register with Ofsted on the voluntary part of the childcare register.
They will need to complete a Common Core course. This could be something like an introduction to childminding course which may be run by your local authority. It could be a distance learning course, such as Common Core Skills (option 1 or option 2).
They will need to do a paediatric first aid course that is approved by the local authority.
They will need to have Public Liability Insurance. This is provided by various companies, including Morton Michel and NannyInsure. Insurers will decide based on their own criteria as to if they will provide insurance or not, so if your au-pair has little childcare experience you may find some insurers decline to insure.
Your au-pair will undergo a CRB check as part of the Ofsted registration. They will probably need something from Poland with regard to their criminal record.
Once your nanny has their registration certificate from Ofsted, they need to contact your voucher provider with the registration details. Then the voucher company can issue you with vouchers. You au-pair will need a bank account. I don't know if a Polish bank account could be used, or if it would need to be a UK account.
Hope that is of some help. It could take a couple of week, to many months to become registered.