Rule #1 of Clarinet Transposition – Always transpose from the concert key to the key it is on clarinet before you do anything else!
This keeps you from getting really, really confused in the long run.
Transposing from a given concert pitch (a C instrument) to the pitch on clarinet (a Bb instrument) is simple. When you’re on clarinet, you play the note one whole step above the concert pitch.
The way I explained it to myself was that, well, what does a “Bb instrument” mean? It means that when a piano plays Bb, we play our C. When a piano plays C, we play our D. So I think of it as because we’re pitched one whole step lower, we need to compensate for that by always playing one whole step higher.
That’s all there is to it. Just make sure that you always transpose before playing, because while not every instrument is in Bb, all of the things you need to learn about music (scales, intervals, note names, anything really) apply to every single instrument. If you know how to transpose, you can work with people playing different instruments and be much less confused.