My opinion is just don't.
It can cause all kinds of weird problems (and performance issues) if you do it.
However, if you still want to, you need to go into settings-->apps-->specific app and see if it will allow you to move some or all of it to the SD card. Some won't, some will.
If it was an app preinstalled with the phone, it will not let you move to the SD card.
And, if you have issues with the app, you'll probably need to move it back to internal storage.
But, really, I'm not a fan of moving apps to SD card. I've seen some of the oddities people "live with" just to have that extra free space. It's not worth it to me.