The use of 2NT as an overcall saying that you have the 2 lowest unbid suits (cf last week) is fine if that is what you have, but if you don’t you're going to need another very popular convention –the MICHAELS CUE-BID.
After an opening bid of 1H on your right, 2NT would say you had both minors. With this hand you have the other major (Spades) and a minor. To tell partner this divine, glorious, helpful news you simply bid 2H! In this situation a direct cue-bid of the opponent’s suit says “I’ve 5 of the other major, and a 5 card minor as well. Any ideas?”
The point count will be in the 5-10 range, depending on things like suit quality, vulnerability, whether partner’s passed already and how much you’ve had to drink. Apart from telling partner useful stuff, the bid also attacks the opponent’s space and crucially makes it absolutely impossible for your left hand opponent to bid 2H! Great. You tell partner about two suits AND you get in their way as well.
Michaels works just as well over a minor suit opening. If opponents open 1C then Michaels (1C - 2C!) says you hold both majors. How useful is that!
The basic hand shape for these bids is two suits each at least 5-cards long. They can be longer (6 - 5, 6 - 6) but never shorter.
Both the Unusual 2NT and Michaels are pre-emptive weakness bids, so if you have a genuine opening hand you must bids your suits naturally. The point is you don't want to eat up your own space when you need a dialogue.
Responding To Michaels
What about responses? Let’s assume partner has gone 2H over the opponent’s 1H. This was the hand remember:
With 3 card support or better for spades, partner of the Michaels bidder can bid 2, 3 or even 4S before it gets back to the opener’s turn. By which time it's all got too high for them to do anything safely.
Without 3 card support for Spades, but a tolerance for either minor, partner bids 2NT. This says “Tell me your minor.”
1H - 2H! - P - 2NT
P - 3D
Sometimes the inevitable happens and it is partner, not the opponents who has the big hand. No problem, partner can force by cue-bidding again!
1H - 2H! - P - 3H!
Now with a heart stop (A, or Kx) you could bid 3NT and see what partner does next. If not, just bid your minor.
Both the Unusual No Trump and The Michaels Cue Bid conventions are so popular and widespread that they are almost assumed to be “standard”, so you’ll come across them again and again.
Have a jolly time!