Given a debian source package - How do I install the build-deps?

Use mk-build-deps which is part of devscripts.

Here is an example workflow using ptman's suggestion.

Assuming you have downloaded the .dsc file, the .orig.*z file, and maybe also a .debian.*z file, then unpack the source package with:

dpkg-source -x [package_name]_[package_version].dsc

Move into the extracted source folder and run:

sudo mk-build-deps -i

This will create the file [package_name]-build-deps_[version].deb and install it, and also install all the dependencies it suggests.

Now you should be able to build your package from source:

debuild

When you are finished, you can easily remove the dependencies that were pulled in, by removing the build-deps package which you generated:

sudo apt-get purge --auto-remove [package_name]-build-deps