To start off, we need to install some packages:
apt install build-essential git apt install dovecot-dev libicu-dev libxapian-dev libsqlite3-dev
Then let's clone the fts-xapian git repository:
git clone https://github.com/grosjo/fts-xapian cd fts-xapian
autoreconf -vi ./configure --with-dovecot=/usr/lib/dovecot make sudo make install
"/usr/lib/dovecot" is the base directory of /usr/lib/dovecot/dovecot-config, installed by dovecot-dev. Update your dovecot.conf as described.
And finally, if it's not a fresh install
# Just index all mailboxes of all users doveadm index -A -q \* # If you want to know what's going on and what "index" is doing: doveadm -Dv index -u $DOVECOTUSER \* # or doveadm -Dv index -u $DOVECOTUSER INBOX