When a partnership does not play negative doubles or Lebensohl to combat opponents' interference after partner's Notrump opening, stolen bids may come into play. Responder's bids are as follows:

a) Where the opponents have bid the suit responder was going to be, s/he doubles. Thus, for example, where opps bid diamonds and responder was going to bid diamonds to make a transfer to hearts, responder would double to make the transfer. This works for Stayman as well.
b) When the opponents have interfered, but have not taken responder's bid, responder makes the bid at the cheapest level. For example, if opps overcall 2D and responder wishes to transfer to spades, s/he makes the standard 2H bid. If responder wishes to bid Stayman, s/he would bid 3C (that's the cheapest level to make the asking bid).
c) other than the 3C bid where Stayman is appropriate, 3-level bids are natural and forcing. (a few don't play it as forcing, but this appears to be the exception)