To become a good you defender you must

Be a very good declarer.

Count, count, count …..
declarers points
declarers distribution
declarers tricks
declarers loser
your tricks
partners points
partners distribution

Agree with your partner upon easy to remember signals for attitude; length and suit-preference

Condition is to be aware that signals are only the second. First is to elaborate what declarer is up to, count, count, count ...............