Hello!
In order to receive training, you must be in the recruiting process for IFATC. This can be done by following the instructions on the post you linked. Before you can start the process, though, you must meet the listed requirements.
After you have applied, you must then attempt a written test. If you pass the written test with an 80% or above you qualify for IFATC training and will work with a trainer to develop your skills.
Before you become IFATC, you will have to be fully familiar with the interface and all the required commands and instructions. A lot of this learning will happen when you have obtained your 500 operations (these are given through controlling on the training server). You will then gain even more skills when working with a trainer. Before you can control on the expert server, though, you will be given a practical controlling test by your IFATC Recruiter.
If you’d like to gain experience and operations before applying for IFATC, I suggest making an ATC tracking thread. You can learn all about these threads at this link:
Let me know if you have any questions!