Hacking EarthBound is made easier by several tools and extensive documentation. Here are a few of them (more can be found on the PK Hack website).
EarthBound Patcher is a ROM patcher for EarthBound, used to create and apply hacks. Since it is written in Python, it is cross-platform by nature; binaries are also available for some platforms:
OS X: EBPatcher.dmg (Currently unavailable)
Linux/Other: EarthBound Patcher must be manually run with Python 3; follow the installation instructions on the EarthBound Patcher page.
EarthBound Hacking 101
EarthBound Hacking 101 is a documentation project by Lyrositor for EarthBound. It covers the basics of EarthBound hacking.
Download: EarthBound Hacking 101 (latest edition)