Go to MyHealth.be, MijnBurgerprofiel, MyHealthviewer or Helena. Log on with one of the digital keys. Itsme is the easiest option. The website will show your certificates. You can download and print them. Be sure to print the certificate of full vaccination (dose 2/2)
You can also call the help desk 24/24 on 078 78 78 50 (in Flanders) or 02 214 19 19 (in Brussels) to have your vaccination certificate sent to you by post.