Obtaining 0x611 or 0x615
This tutorial can be achieved on real hardware, or on mGBA emulator using an original (dumped) GBA bios. Other emulators might not be accurate enough.
First of all, you have to get an egg of species 0x611 (for European versions) or 0x615 (for Japanese version) in order to achieve ACE.
For that, you can follow this tutorial by Rileyk64. This tutorial is for getting 0x611 (European versions),
but it can be adapted for the Japanese version.
In order to get 0x615 in the Japanese version, you just have to use the following EVs requirements: 21 HP EVs and 6 Attack EVs
.
With 0x611/0x615, you can already execute a lot of ACE codes. In particular, a lot of codes can be found on Sleipnir's Pastebin. Make sure to have copies of your 0x611/0x615 egg before hatching one! You can also find codes on the ACE generator, but some of them will require following the next two tutorials in order to setup the exit code bootstrap (it is specified when it is the case).
If you want to go further, allowing faster and more powerful ACE, you should follow the next tutorials. Make sure you have several copies of the 0x611/0x615 egg (3 should be enough) before proceeding.