After 1NT-2C-2D, responder then tells opener which 4-card major he is interested in, and opener can either raise or go back to NT. Responder tells opener he has a 4-card major by bidding the other major, thus 1NT-2C-2D-2H shows 4 spades and 1NT-2C-2D-2S shows 4 hearts. Responder can show both 4-card majors by 1NT-2C-2D-2NT.
Here is a typical Puppet Stayman auction
1NT-2C-2D-2S-3NT.Note that opener has denied a 5-card major and has denied a 4-card heart suit by not raising hearts after responder bid 2S, but opener's bidding has not revealed anything about whether he has a 4-card spade suit as he would have had to do in a standard Stayman auction. This may be an advantage during the play in 3NT.