I'm a new boy at transposition school(!). (I play piano so am familiar with which note is which in the two clefs.)
I play clarinets, Bb and A. We only use treble clef, so it's a simply process of raising the key by a two semitones (Bb clarinet) or three semitones (A clarinet) to accompany our trio who play in concert pitch.
However, my other band needs help transposing trombones and tubas from bass clef to treble clef. I have read the instructions above, but I need to understand the 'why' before I can help the band, and spot if I am transposing the tuba or trombone correctly. Obviously I don't need to understand brass instrument fingering, or trombone positioning, but must grasp what happens for the change from bass clef to treble clef.
I used to think that, if a brass instrument should play a 'C' in bass clef, then, (after using Musescore to transpose), the instrument should play a C in the treble. Not so, it seems. That is my mystery.
Any pointers for my now-addled brain would be most welcome.