Instructions
Gain access to Pal Park. You can walk to Pal Park at any time during gameplay, but you cannot get in until you accomplish a few prerequisites. To get in, you will need to have gotten the National Pokedex, then gone out into the world and seen every Pokemon in Sinnoh.
Travel to Pal Park, which is north of Fuchia City. In pre-Ninetndo DS games, Pal Park used to be the Safari Zone.
Find the Warden outside of the Pal Park office and speak to him. He will give you a tutorial in using Pal Park to import your GBA Pokemon to the current game your are playing.
Find and capture your Pokemon. Importing Pokemon from your GBA Pokemon game does not add them to your inventory, it instead releases them into Pal Park. You will have to hunt them down, battle them and capture your old Pokemon again. These battles can be difficult, because your Pokemon will be at the level and have the abilities you trained them up to in the previous game.