How do you transpose piano music to Bb tenor sax?

This depends on what key the Trombone is in and what clef the music is written in. Most trombone music in treble clef is for Bb trombone so there would be no transposing needed, they can play straight from the music. However, if they only read bass clef then you need to effectively 'transpose' the music down a third (two notes below on the stave)