I have a cello part written in bass clef. I want to transpose the part to be played by a B flat bass clarinet who reads only treble clef. How? For example, a G (being the bottom line on the bass clef) on the cello part in the key of C would be what note for the B flat bass clarinet?
It's not too hard. The bass clarinet is in Bb meaning that when the bass clarinetist plays a "C" it come out as "Bb." It also is written an octave hither than it sounds is written. This means that the when the bass clarinet plays "middle C" the sound is the Bb below the C an octave below middle C. In other words the bass clarinet sounds a ninth below its notation. To transpose a cello, one moves all cello notes a ninth upward and changes the key signature to two fewer flats or two more sharps or the equivalent.
I'd do this in three steps (though some of these can be combined.) Transpose the cello part up one tone in its own clef and change to the proper key signature. Then move all the notes up an octave. Then rewrite these notes in the treble clef.
Actually, many music writing programs can do the for you, but its not particularly hard to do by hand. Many bass clarinetists can also do the transposition on the fly.