To be completely honest, it's just a matter of practise and summoning your Persona repeatedly. Eventually, you'll get use to the feeling and it'll be easier to summon them, or at least that's how it was for me.

[ might be different for him since he doesn't SHOOT HIMSELF IN THE HEAD or RIPS OFF HIS FACE MASK ]

I would recommend breathing exercises and having someone with you when you practise in case you overstrain yourself.