Secret Bases are small special areas in the Gen 3 Hoenn games that the player can decorate and use to battle their friends using the record mixing feature. This is a simple guide to building your own Secret Base and collecting all the decorations in the game. Collecting all the decorations and making a cool Secret Base is an entirely cosmetic achievement, you don't get any rewards for it or anything, but I mean, it's Pokémon fan culture to spend unholy amounts of time on cosmetic stuff. Just look at shiny hunters.
I'm focusing on the decorations right now because it's the most time consuming aspect, but I'll write about finding a good Secret Base spot and something about the mixed record trainers soon.