how does training special ability work? Worth it?

I want a player of mine to gain a special ability, namely free-kick taking or corner taking. But I don't really know how this training of a special ability works. How long does it take and can I train the player normally in other things during this time? Does it cost any tokens?

Also - is it worth training a special ability? In the old format of Top Eleven, it wasnt really worth it considering how much you lost in normal training just to gain a special ability

No one knows how do SAs work and is it worth it.

Also, read this.