Sometimes there is a bug in a package from ubuntu that is not yet fixed, or you just want to modify something in the source of a software. Here is an easy way to build your own package from the ubuntu sources.
First you need to install the required tools for building software from source
sudo apt-get install build-essential
Let's assume you want to build a package called foo and apply a patch bar.patch to it.
First you have to download the necessary build dependencies for foo
sudo apt-get build-dep foo
Now you are ready to download the actual sources of the package (Don't use sudo here!!)
apt-get source foo
This gives you a directory called foo-<version> you can now cd into it apply the patch and build the package
cd foo-<version> patch -p1 < bar.patch dpkg-buildpackage
Now you have a package called foo-<version>.deb in the same directory where you have run apt-get source. You can install it using dpkg.
sudo dpkg -i foo-<version>.deb