This link explains how to do this but you’ll find that as you become more experienced at playing (I believe you said you’d only recently begun to play clarinet?) you will be able to do it naturally, as you play.
Get the piano music for a song that you know really well and find the first note to play – you’ll be able to hear when it’s the correct one – then with a little practise you’ll be able to (most of the time) just go up or down the same spaces as the first note was.
It won’t always work with every note but you’ll hear when you are wrong if it’s a tune you are familiar with.
Transposing a Melody for a Transposing Instrument ,
You might have to transpose a melody into (or out of) concert pitch. In Grade 5 Theory, you will always be told which direction you have to transpose in (up or down), and by what interval (major 2nd, perfect 4th etc.) (You don’t have to work out from scratch how to write out a piece of clarinet music so that it sounds at concert pitch, for example!)
You will need to understand intervals properly before you can begin. (Have a look at “Lesson 7 – Intervals” if you need to). Sometimes you are asked to use a key signature, and sometimes not. Read the question carefully!
Start by carefully transposing the key signature, if you have been asked to include one.
For example, if the key signature is A major (3 sharps) and you have been asked to transpose down a major 2nd, you will need to write the key signature for G major (1 sharp).
Don’t forget to add the time signature (this doesn’t change, of course):
(The diagrams are visible on the link).
Next, transpose each note in turn. Be careful when you come across accidentals - in the above extract the first accidental is E sharp. Transpose E sharp down a major second, and you get D sharp.
(If you think that E sharp on a piano keyboard is the same as F, you might think the correct transposition would be E flat - but you would be wrong: E sharp - E flat is actually an interval of a double-augmented tonic!)
Here is the finished transposition:
This link is helpful as well.
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