Transposing sheet music to fit the ocarina
One thing that you are going to run into when playing the ocarina is sheet music that is out of range. A single chambered alto C ocarina can play from about C to F (excluding subholes), which in sheet music looks like this:

You can find the range of a piece of sheet music by scanning through it, looking for the highest and lowest note used.
You may however run into a piece of music which as written has a range of G to C on the second ledger line.

On first impression you may think that this to be unplayable. But it actually can be played using a technique called transposition. Transposition allows you to alter the music, raising or lowering its pitch to bring it into the range of your instrument.
How does transposition work?
Transposition relies on the fact that all of the major scales (or keys) are based on the same pattern. For example, if you consider the notes of both the C Major and D Major scales:
C, D, E, F, G, A, B, C
D, E, F#, G, A, B, C#, D
Both are formed from the chromatic scale following the formula:
Whole, Whole, Half, Whole, Whole, Whole, Half
Each note in both of these scales can be given an equivalent number.
- For the C scale, C would be 1, and D would be 2 etc.
- For the D scale, D would be 1, and E would be 2 etc.
If you play a piece of music using the same numbered notes from two different scales, It will sound like the same tune despite being higher or lower in pitch.
To transpose a piece of music just means substituting the notes in one scale, for the note of the same number in a different scale. Transposing from C Major to D Major for instance, the notes C D E turn into D E F♯.
Another way of thinking about this is just moving all of the notes in a melody up or down by some number of semitones within the chromatic scale, as demonstrated by the tool below. The 'transposition' slider moves the notes, and you can press play to hear how it sounds.
Transposition as a concept is extremely simple as you can see, but the nature of instrument fingering systems, and other things like music notation can make it look much more complicated than it is.
How to transpose music
From the above explanation, technically you know how to transpose music, substituting the note in one scale into the equivalent note of another scale. If you search for "music transposition table" on the internet, you will find people have already made tools to help you do this.
However, transposing music one note at a time, and writing it out in the new key is needlessly difficult and time consuming. There are much easier options.
Computer aided transposition
First, if you are using a computer readable music format like ABC notation, MusicXML, or a score editor, almost all of these tool have a feature that lets you transpose the music by just selecting the new key from a drop down menu.
Transposing music at sight
If you remember that all of the scales are based on the same pattern, it is easy to learn how to transpose music at sight.
If you consider this:

And the same music transposed into C:

Do you notice that the movement between the notes are the same? In both examples:
- The first note to the second note is 1 staff position.
- The second to 3rd note is one staff position.
- Third to fourth is 5 staff positions, etc.
On an alto C ocarina, you can thus 'rename' the 'G' line on the staff to 'C', start reading by the differences between the note positions, and the result will sound correct, transposed to C.
The only things that you need to consider when doing this is the difference in the key signature between your instrument, and the sheet music, and where the tonic is within the scale you are reading.
This same technique can also be used to sight-read music at written pitch on an ocarina that is in a different key, which is discussed in the article An easy method of playing ocarinas in different keys.
Closing notes
Transposing music becomes easy with a bit of practice, and opens you the ability to play sheet music that you may have thought was out of range.
It is a good habit to start viewing sheet music with an eye for the range of notes used, as transposition doesn't mean that everything will fit. See Identifying ocarina friendly sheet music.
Multichamber ocarinas help, and sometimes, music can be modified to fit into a smaller range, which is discussed in Modifying music to fit the ocarina.