It is assumed you've already setup your build environment and cloned the repositories.
The following steps allow you to build - and run - B2G on your emulator. Do NOT run them as root.
$ cd B2G
$ make sync
Build the configuration for QEMU:
$ make config-qemu
Next, build the gonk backend and then the system itself:
$ make gonk
NOTE: if ./emu.sh - your emulator - doesn't run at first, you'll need to make it executable:
$ chmod 777 emu.sh
Lastly, run the emulator:
NOTE: After cloning your repositories or making your build the first time, you can "clean" them - e.g. put them into the state so that the latest pull is the same as a fresh clone.
To do this, from your repo root:
$ make sync; git clean -xfd; git submodule foreach "git clean -xfd"
Alternately, you can try:
$ make mrproper