Count your bid
Start with sure tricks: the ace of spades, the king of spades with the ace or two more spades beside it, and side-suit aces. Every spade beyond your third is worth about half a trick, because spades win once the others run out, and with three or more spades a void or a single card in a side suit adds half a trick, since you can trump it. A side king counts half a trick with one or two cards beside it; alone, or in a long suit where it is often trumped, it counts nothing. Partners' cards make a hand take a little more than its sure count, so the suggested bid in practice is the count plus a half, rounded: 2½ sure tricks suggest 3, three suggest 4.
When to bid nil
A nil hand has no aces, no more than one king, two or fewer spades with none above the 7, and a card of 5 or lower in every side suit so you can always play under. Long suits are fine if they are low. One high spade is the usual way a nil dies: you cannot get rid of it until spades are led. The practice nil-risk line shows how often your hand survived nil in simulated deals.
Cover your partner's nil
When your partner bids nil, your job is to take the tricks they might win. Lead high cards so your partner can play under them, play high before your partner when you can, and overtake a trick your partner is winning. The extra tricks you take become bags, which is a fair price for 100 points.
Set a nil
Against a nil, lead low cards in suits the nil bidder still holds and keep your own low cards so you can play under them later. When the nil bidder is winning a trick, play underneath and let the card stand.
Avoid bags when you are ahead
Once your side has its bid, extra tricks are bags. With seven or more bags, duck tricks you do not need: play your highest card that still loses. Sometimes taking a bag is still right, when it sets the other side's bid.
Pull spades and read the meter
With four or more spades, or once your contract is safe, lead spades to draw the other side's trumps so your side suits can win. The over/under meter on the table shows the total of all four bids against 13: under means spare tricks (bags) are coming; over means at least one side will be set, so fight for every trick.