First install the build dependencies:
sudo apt-get install u-boot-tools snapcraft crossbuild-essential-armhf lzop snapd ubuntu-image
First register a key with snap as shown here.
Create a file in the root of this repository called "config.inc". Add your authority id, and optionally a separate brand id. This can be found on your ubuntu account here
This file should contain something like:
AUTH_ID="longstringofcharacters"
Finally, just run:
./build.sh
This will compile the kernel, gadget snap, and assemble the base ubuntu core image in output/