Here is an SATB hymn tune Pansies, Lilies, Roses which is not decorated:
To make it a little more rhythmically and melodically interesting, we can add unaccented passing notes between any notes which are a 3rd apart. (Remember, never add a decoration note between the leading note and dominant however!) Unaccented passing notes are the most commonly used type of melodic decoration.
For example, in the alto, the first two notes (D-F#) are a 3rd apart, so we can add an unaccented passing note E, and there are other places in the alto and tenor where passing notes work well.
When adding passing notes, always check for consecutive 5ths and octaves, and avoid augmented 2nds in minor keys.
Auxiliary notes are used much less often than passing notes and often they do not add any beauty when used by themselves. If you are unsure whether an auxiliary note will improve your harmony, it usually best to leave it out!
(In the ABRSM grade 6 exam you are not specifically required to use melodic decoration in the harmony questions, but you are required to write an interesting soprano line, and melodic decoration might help you achieve this.)