Many singers think improvising and phrasing are difficult and feel like they are on shaky ground when they improvise or make phrasing changes. Some even think that these are natural abilities that you either have or do not have. They believe you cannot learn them and therefore will never be a good singer or musician. This is of course nonsense! Improvising and phrasing can be learned and requires practice like anything else
Some singers start by singing something casual and from this select what they like.
Some singers adopt the phrasings of other singers and put parts of these phrasings together to create their own.
Some singers study music theory and start improvising on the basis of a theoretical harmonic overview.
Most singers take something from each method and just start Improvisation.
A lot of improvisation and phrasing use Pentatonic and Blues scales.
It would be good to practice them!