Playing Null
Null throws out everything else in Skat. No trump, no points to bank, just one job: lose every trick. The cards run 7-8-9-10-Jack-Queen-King-Ace, with each Jack sitting in its own suit. Every card you play as declarer, lead or follow, is aimed at that single goal.
Read the official rule on Null's card order.
Out of the led suit and free to discard? Throw the Club Ace before the King. Both are stranded up top, but the Ace is the one card that can never slip under anything later.
The one lead you might get
As Null declarer you lead at most once: trick 1, and only when you're sitting in Vorhand. After that you are always following, because leading again would mean you had already won a trick and lost the game. A safe lead needs two things at once: nothing outstanding below it in its suit, so any defender still holding that suit has to play over you, and something still outstanding above it. A suit nobody else has left takes the trick for you.
Ducking under
Following suit, the first question is simple: does a card in your hand stay under what's already on the table? If it does, play it. Ducking costs nothing and keeps you exactly where a Null declarer wants to be, still on zero tricks.
Dumping a liability
Out of the led suit, you're free to play anything you hold. That's the moment to get rid of your most exposed card: the one with the fewest cards still out above it in its suit, because that's the one with the least room left to duck under later. What stays in your hand counts as much as what goes, so the card to throw is the one that leaves the safest hand behind.
When you're forced
Sometimes every legal card in your hand wins the trick. That's the Null gone: one trick loses the whole contract, and there is no cheaper or dearer version of losing it, because Null counts no Augen at all. Nothing is left to protect, so put your lowest card down and play the hand out.