Catch Shaymin on Pokémon Diamond or Pearl

This is how to catch Shaymin.^[1] You don't have to have an Action Replay, but it makes it faster.

Steps

Go to Victory Road.
Find an exit tunnel that leaves to the right.
Follow the path until you get to Shaymin. It is very weak, so go easy.^[2]Use "Walk Through Walls Cheat" to go to the silver rock by Victory Road.
Go forward a bit, once you see the first Shaymin, it's just a mirage so use your bike and keep going forward through the flowers until you see Shaymin. An easy way to catch Shaymin is using the 100% catch code on your Action Replay. The code is: 9223c1f4 00002801 1223c1f4 00004280 d2000000 00000000

Go to Victory Road.
Near the exit, there should be an entrance where a man was blocking it. Go through there, and you should meet a Trainer who will help you fight through the tunnel.
When you reach the exit, explore until you get to the Silver stone.
Press A to talk to it. Afterwards, Prof. Oak will come and will ask you what you would like to give thanks for. Enter a phrase then confirm.
The stone should transport you to where Shaymin is.
There will be a long, ongoing road. Use your bicycle to quickly navigate to the end.
Shaymin should be there. Press A to talk to it and a battle will start. Shaymin will be at Level.30.

Save before you battle it.
If you have an Action Replay, use the "Walk Through Walls Cheat". Go to the league and walk to the side. You will have to walk through a bunch of water, but you will eventually make it.

The use of a cheating device such as an Action Replay can result in data corruption or can result in the DS freezing.
If you use the Action Replay, there is a high chance that you will get lost. It's probably better not to cheat.