Install sbuild:

sudo apt install apt-cacher-ng sbuild schroot debootstrap
sudo adduser sbuild $(whoami) # you might need to re-login after this

Create a trixie sbuild chroot:

sudo sbuild-createchroot --include=eatmydata,ccache trixie \
    /var/lib/sbuild/trixie-amd64-sbuild \
    http://127.0.0.1:3142/debian

Same procedure applies for unstable by just replacing trixie by unstable.

Then on a regular basis (or in crontab) you might need to update the sbuild chroot environment:

sudo sbuild-update --update --dist-upgrade --clean --autoremove --autoclean trixie

Build a source package:

dpkg-buildpackage --build=source --no-check-builddeps

Build a binary package:

sbuild -d trixie