>>553107842/3 (apparently)
Now download and open EonTimer. Whichever version of it that works on your computer is fine. In the settings tab in Eon Timer select the game and the way you're playing it. If on a GBA/SP select GBA, if on NDS select GBA/NDS, if emulator select that. Now save the settings and close that window and copy the advance you want into the target frame box in the main screen of Eon Timer. By default it will produce 6 beeps on a 5-second countdown, this is the "pre-timer", a countdown for you to soft reset the game to start the seed at the right time. Then press A quickly to load your save and then press Start to keep any NPCs from moving, as NPC movement accelerates frame advancement. You'll want text speed set to Fast.
Now you wait, and keep an eye on the countdown in Eon Timer. When it gets back down to 2 seconds or so it'll beep again 6 times. The final beep is when you should trigger your encounter, so keep in mind when you need to close the Start menu, or go through dialog boxes. After that, catch the Pokemon. A Master Ball is helpful, you'll be soft resetting after so you'll get it back. Don't save at any point after this. Your caught Pokemon won't be shiny, but you need to figure out its IVs to see which frame you hit to adjust EonTimer's calibration to get closer to your target. It helps to have an open party slot and a bunch of Rare Candies for this, Emerald has a cloning glitch you can exploit the fuck out of. There's IV calculators online, and Pokefinder has one built in too. On the main window where you select a generation choose Tools at the top, then IV Calculator. You can also teach that Pokemon Hidden Power and figure out what type it is through trial and error to help determine your IVs.