When you are the declarer in bridge, one step in planning the play of the hand is to count your winners or losers. In no trump, it is often recommended to count winners and in a suit contract, to count losers. Once you have counted your losers, you can see how many you need to eliminate to make your contract and then try to figure out how to do so.
Determine which hand (dummy or declarer) has the most trump. (This will almost always be declarer's hand). Call this the long hand.
Determine the number of losers in the trump suit. In the trump suit, losers are the number of top cards you are missing and cannot remove. For example, if you have AKQ of trump and a total of 10 trump cards in the two hands, that leaves three for the opponents. Even if they split 3-0, you will be able to draw them all, so you have no losers. But if you have AKQ of trump and only nine trump cards in your two hands, you might not be able to draw the J, so you have one loser.
Determine the maximum number of losers in each non-trump suit. This is the number of cards in that suit that are in the long hand.
Reduce this maximum by looking for high cards in each suit. The cards that count as "high" in each suit are the number of cards held in that suit in the long hand. For example, if the long hand has three cards in a suit, then the high cards to count in that suit are AKQ. If the long hand has only one card, only the A will count as high in that suit.
Add up the number of losers in each suit and this is your total number of losers.