To use this secret, you need to have either beaten the main game or be able to recycle for reviver seeds at Spinda's Cafe. So...

If you've beaten the game, simply go to the Guild, sublevel 2. In the bottom left-hand corner, you'll see Loudred standing by a hole. Talk to him, and he'll ask you if you want to do sentry duty. Say yes. Complete the minigame, then reap the rewards. If you got all six questions correct, but you didn't beat your high score, you should get a Def. Scarf, a Heal Seed, 300P, and.... a Reviver Seed! Repeat this several times and watch the seeds flood in.

If you haven't beaten the game, but you can recycle for a reviver seed at Spinda's Cafe, go to Marowak Dojo. Enter one of the mazes. It helps if you have a type advantage, but it's not essential. If you have a pokemon who knows the iq skill Acute Sniffer, this is also useful. However, remember not to take in any items. If you have Acute Sniffer, you should be told how many items there are on the floor. If you don't, just search every room until you have all the items. Conveniently, the only items you'll find in the mazes are Oran berries, Escape Orbs and money. When you've completed all five floors of the maze, exit the dojo, bank your money and go to Spinda's cafe. First stop, the recycle shop. You can recycle two escape orbs for one reviver seed, so go ahead. Then, go to the juice bar. If you drink enough oran berry drinks, your hp might get boosted! Then go back to the maze and repeat as many times as necessary.

Hope this helped!