Based on article by Barbara Seagram "What to do when partner opens 1NT and your RHO interferes". Visit her website to find more resources.

Barbara Seagram writes:
This has become more of a challenge in the world of modern day bridge. Much of the time, your RHO will be making a bid that has an artificial meaning e.g. Opponents are playing D.O.N.T. or Cappelletti or Meckwell. Often they are showing 2 suits, one of which may be an unknown suit. It is essential that you discuss your responses with partner.
The following is a simple approach that assumes that you are not playing Lebensohl (a very good convention but a complicated one).

Why not to use "Stolen Bids" after ALL interferences? "Stolen Bids", though common among club players, is definitely inferior and should not be used by any player at all. Larry Cohen said, diplomatically, about "Stolen Bids", something like, "I never used it, recommended it or would teach it." Jerry Helms considered it basically total trash.

The following assumes that the "system is on" only after Double or 2♣, regardless what Double or 2♣ means. After ALL other bids the system is off, except 4-level Texas transfers.

1NT - Double

• Redouble
  7 or more HCP. "We have more points than they do". O.B. (Opening Bidder) must pass.
• 2♣
  Stayman
• 2♦
  Transfer to hearts
• 2♥
  Transfer to spades
• 2♠
  Transfer to clubs
• 2NT
  Transfer to diamonds

1NT - 2♣

• Double
  Stayman. This is the ONLY "stolen bid" you should use.
• 2♦
  Transfer to hearts
• 2♥
  Transfer to spades
• 2♠
  Transfer to clubs
• 2NT
  Transfer to diamonds
• 3NT
  10-15 HCP. Your hand is not suitable for Stayman. You may or may not have a ♣ stopper.
• 4♦
  Texas transfer showing 8+ HCP and 6 or more hearts. Partner has to bid 4♥.
• 4♥
  Texas transfer showing 8+ HCP and 6 or more spades. Partner has to bid 4♠.

1NT - 2♦

• Double
  If 2♦ is for majors, it shows diamonds. Other wise, 7+ HCP value-showing. O.B. may pass for penalty.
• 2♥
  5-9 HCP + a 5 card heart suit
• 2♠
  5-9 HCP + a 5 card spade suit
• 2NT
  8-9 HCP, inviting O.B. to bid 3NT with a maximum or pass with a minimum. You may or may not have a D stopper.
• 3♣
  10+ HCP and a good 5 card club suit or a 6 card club suit. You don’t have a 4 or 5 card major. Forcing.
• 3♦
  Stayman. 10+ HCP.
• 3♥
  10+ HCP and a 5 card heart suit. With 6, you would have used Texas. Forcing.
• 3♠
  10+ HCP and a 5 card spade suit. With 6, you would have used Texas. Forcing.
• 3NT
  10-15 HCP. May or may not have a diamond stopper. No 4, 5 or 6 card major.
• 4♦
  Texas transfer showing 8+ HCP and 6 or more hearts. Partner has to bid 4♥.
• 4♥
  Texas transfer showing 8+ HCP and 6 or more spades. Partner has to bid 4♠.

1NT - 2♥

• Double
  7+ HCP, value-showing. This is not a stolen bid. O.B. may pass for penalty.
• 2♠
  5-9 HCP + a 5 card spade suit, to play.
• 2NT
  8-9 HCP, inviting O.B. to bid 3NT with a maximum or pass with a minimum. You may or may not have a H stopper.
• 3♣
  10+ HCP and a good 5 card club suit or a 6 card club suit. You don’t have a 4 or 5 card major. Forcing.
• 3♦
  10+ HCP and a good 5 card diamond suit or a 6 card diamond suit. You don’t have a 4 or 5 card major. Forcing.
• 3♥
  Stayman. 10+ HCP. You have a 4 card spade suit.
• 3♠
  10+ HCP and a 5 card spade suit . With 6, you would have used Texas.
• 3NT
  10-15 HCP. May or may not have a heart stopper. No 4, 5 or 6 card major.
• 4♥
  Texas transfer showing 8+ HCP and 6 or more spades. Partner has to bid 4♠.

1NT - 2♠

• Double
  7+ HCP, value-showing. This is not a stolen bid. O.B. may pass for penalty.
• 2NT
  8-9 HCP, inviting O.B. to bid 3NT with a maximum or pass with a minimum. You may or may not have a H stopper.
• 3♣
  10+ HCP and a good 5 card club suit or a 6 card club suit. You don’t have a 4 or 5 card major. Forcing.
• 3♦
  10+ HCP and a good 5 card diamond suit or a 6 card diamond suit. You don’t have a 4 or 5 card major. Forcing.
• 3♥
  10+ HCP and a 5 card heart suit . With 6, you would have used Texas.
• 3♠
  Stayman. 10+ HCP. You have a 4 card heart suit.
• 3NT
  10-15 HCP. May or may not have a spade stopper. No 4, 5 or 6 card major.
• 4♦
  Texas transfer showing 8+ HCP and 6 or more hearts. Partner has to bid 4♥.