For the instruction, you must either make an appointment with your local health authority ("instruction in presence") or you can make use of the option to be instructed online ("online instruction"). After the instruction - both in person and online - you will be issued with a certificate of attendance.
After the instruction, you must declare in writing that you are not aware of any facts that would prohibit you from working.
You will then receive the certificate of attendance.
If there is a suspicion that you cannot obtain the certificate, for example in the case of a food-borne illness, you will only receive the certificate once a doctor has ruled out the suspicion of illness on the basis of an examination.
The content of the certificate is that you have been instructed about your legal obligations, in particular about which infectious disease you are prohibited from working in the food sector.