Do a google search for the base rom the rom hack is based on, in this case a normal copy of Pokemon Ruby. For legal reasons, we cannot provide you with a link to a download to that.
Then you want to download 2 programs. Visualboy Advance is the big one as you need it to play the game.
The second program you need is an IPS or UPS patcher like IPSwin, Lunar IPS, or Tsukiyomi, among others.
Then you download the IPS or UPS file in the first post of a hacks thread. The first post will contain either the required file in the post as an attachment (due to problems with the site being hacked a while back, not all of them work) or will provide a link to download the file in question. It is legal to share IPS and UPS files, but not rom files.
Copy the base rom into a new folder specifically for the hack. Now open your patching program (I like IPSwin for IPS files, but many here prefer Lunar IPS). Select the rom base in the hack's folder, and then select the IPS file and patch it. Now rename the patched game to match the name of the IPS file, but keeping the original extension (like say you patched a Pokemon Red Rom into the hack Pokemon Brown. The IPS file would likely be pokemon_brown.ips. So you rename the newly patched rom to "pokemon_brown.GB". The file extension for roms will generally be .gb, .gbc, or .gba. Don't change the extension or it WILL NOT work.
It sounds more complicated than it is.. The process takes a few minutes at absolute most.