It's with default controls.
You must push L1 first and then,while still holding L1,push R3.The right analog stick that is.Just click it inside once.
In this way the cursor goes to your GK and you can control him manually like any other player.
When you control him,there are some actions you can make with different buttons.
To make a save i think you must press

.

also does something i think.If the shoot goes straight to him he still saves it even if you don't push anything i think.
But even if you don't control him manually,it still helps if you press

as he's about to save or when the opponent shoots from close range.If you time it right your GK reacts better and he's more likely to make the save.
The manual GK is more effective on free kicks etc.When the opponent's free kick animation starts,you instantly take control of your GK and move him to the side of the goal which is more open so he's more likely to save the shoot because everyone online and even the AI will always shoot to the open corner.
But be careful on online.If you do it once,then next time your opponent will know it and he maybe will shoot to the other side to trick you.
If you do it against the AI it's kinda cheating from your side because you'll always save any free kick so i don't use it much.More realistic for me to just push

so the players in the wall will jump.If it goes in,it goes in.
I don't want to cheat the AI like this.Well..if it cheats me badly..then i'll also cheat it back sometimes.