Hi,
Here’s a link to the [https://cardiagn.com/mazda-323-protege-bg-factory-service-manual-1989-1994-pdf/|Mazda 323 Protege Bg Repair Manual 89-94]
Scroll down and click on ''Engine SOHC'' or ''Engine DOHC'' (you didn’t specify) to view the procedure to replace the head gasket. For the SOHC engine it starts on p.B1-16. I didn’t check for the DOHC engine but it would be in the index at the start of the section