for pgtuning you just need to use pgtune.leopard.in.ua as mixed application. The iceshrimp doc could you to understand a bit more what you wanna do.
but when you have generated the pgtune, juste go to
/etc/postgresql/15/main/postgresql.conf and add it wherever you want in the file (I just copy paste it at the beginning)then you just need to restart the DB
sudo systemctl restart postgresql@15-main