I have been practicing volume swelling throughout the years but never did it systematically. It is a bit tricky to play these notes in beat with metronome, so I have to do it in free rythm. A good exercise that I need to do is just to play scale, fading in each note, with very slow metronome beats. The goal is to make sure that you start fading in the volume almost instantaneously when picking the note, so that the first sound comes right with the metronome tick.
I also really like adjusting the volume on the guitar, without touching the effect circuit. This is a very old (1971) Japanese strat, with original pick ups that are very responsive to such adjustments, allowing for variety of sounds.