Defending Null
Defending Null has one job: make the declarer win a single trick, any trick. There is no trump and nothing to count, and the cards run 7-8-9-10-Jack-Queen-King-Ace with each Jack in its own suit. Every lead, follow, and discard below is aimed at forcing that one moment open.
Read the official rule on Null's card order.
Spades is your shortest suit, so the King goes first. Once the Queen follows it, Spades is gone from your hand for good and can never force you to win a trick.
Lead low from your longest suit
Low is the forcing direction: the lower the card you lead, the more of the declarer's holding sits above it, and length keeps that pressure running in one suit until their last card there is forced up and over. Skip any suit the declarer is already known to be void in, leading there only hands them a free discard.
Leading high to probe
When nothing sharper offers itself, lead high instead: your highest card in the suit where the most high cards are still out, or simply your highest card. A high lead only forces anything while cards above it are still unaccounted for. If the declarer can't stay under it, that's the trick forced right there. If they can, watch how close it was, because a card that only just slipped under tells you where they are thin.
Attacking a suit that already cracked
Watch how the declarer ducks. A card that only just squeaked under the rest of the trick, not comfortably, means that suit is thin in their hand. Lead your highest card straight back into it: the safety margin that saved them last time may not be there again.
Leading into your partner's void
Sometimes your partner is out of a suit while nothing so far says the declarer is. Lead exactly that suit: your partner can't follow there, so the whole trick comes down to your card against the declarer's, with nothing else in the way.
Following low
Once you're just following, not leading, the choice is simple: play the lowest card you're allowed to play. Losing the trick costs you nothing, defenders don't need to win, while it saves your higher cards for a lead that actually applies pressure.
Shedding a short suit
Out of the led suit and free to discard? Shed the highest card of your shortest suit and leave your longest alone: that one is your lead-pressure engine, worth protecting. High cards are never useful as a low lead anyway, so this is a clean way to get rid of one while emptying a suit that could never have forced anything. Only when every suit you still hold is the same length does that guard drop, and then you simply throw your highest card.