You just clasp the ball to your chest.That was also mentioned by Bryan King (I think it was him) on the Podcast that was posted earlier. The keepers of the past were commanding the six yard box with catches, as opposed to modern day keepers who are punching or flapping.
It isn't bleeding' rocket science.
I used to work with some keepers at a pro club. We'd spend 20 minutes with me putting balls into that area so the could practice coming and claiming, hands pointing up towards their face (not down, as our man so often does), their bodies arched and head over the ball.
A basic technique.