What is a “useful void”? If the opponents have been silent, it is any suit outside trumps except a suit your partner showed. If they have bid a suit(s), it is their suit.

Method 1

Bid whatever strain you were going to bid, except bid it at the six level instead, but not past six trumps. For example, playing 1430, 1❤️ – 4N – 6♦️ would show zero or three plus a useful void, and 1❤️ – 4N – 6❤️ would show two with a void. You don’t have room to show if you do or don’t have the ❤️Q since you can’t bid past 6❤️.

Method 2

• 5NT an even # of key cards and a void somewhere.
• 6♣️ an odd # of key cards and:
  • a club void if clubs aren’t trumps, or
  • a void in any other suit if clubs are trumps
• 6♦️ an odd # of key cards and:
  • a diamond void if hearts or spades are trumps, or
  • a void in hearts or spades, if diamonds are trumps
• 6❤️ an odd # of key cards and:
  • a heart void if spades are trumps, or
  • a spade void if hearts are trumps

Reference: Roman Key Card Blackwood on bridgebum