Your build environment must be FreeBSD 10.3-RELEASE or later (or FreeBSD 10-STABLE) (building on FreeBSD 9 or 11 is not supported at this time).
An amd64 capable processor. 12GB of memory, or an equal/greater amount of swap space, is also required.
An internet connection for downloading source and packages

Building FreeNAS

Note: All these commands must be run as root.

Install the dependencies:

# make bootstrap-pkgs

Download and assemble the source code:

# make checkout PROFILE=profile_type

Compile the source, then generate the .ISO:

# make release PROFILE=profile_type

The vaid profile types are "freenas9" and "freenas10" (see the build/profiles directory). Instead of specifying PROFILE=profile_type, you can set the profile type in the file build/profiles/profile-setting (e.g. echo freenas9 > build/profiles/profile-setting).

Once the build completes successfully, you'll have release bits in the _BE directory. 😄

Updating an existing installation

To update an existing FreeNAS 9.10 or 10 instance that you are using for development purposes:

make update
make ports
make reinstall-package package=freenas host=root@1.2.3.4

Where 1.2.3.4 is the IP address of your development platform. SSH will be used to push and install the new packages onto that host. (Note previous comment about setting the profile.)