Find an SRPM you'd like to build, either one you've built yourself, or one from the build system
Start a container with a XenServer branch name, zero or more package names, and zero or more SRPM paths like so:
./run.py -b trunk-ring3 -p xapi -s xenopsd-0.10.1-1+s0+0.10.1+10+gf2c98e0.el7.centos.src.rpm
The container will run yumdownloader to download the SRPM for each package specified with -p, run yum-builddep against these SRPMs as well as the SRPMs specified with -s, and drop you into an interactive shell. In the above example, the container will install the dependencies for xapi (as defined by the SRPM in the standard repo) as well as dependencies for xenopsd (as defined by the specified local SRPM).
You should then have all the dependencies to be able to build the components whose SRPM or package name was specified above, e.g.
git clone git://github.com/xapi-project/xenopsd
cd xenopsd
./configure
make