HOLIDAY YASUMI

A boy wants to confess his love to his senpai, but his plans are ruined when he turns into a girl. The twist is when she finds out that he's a girl, and she tells him she used to be a boy too, and promises to show him the ropes of girlhood.

Short manga posted by Holiday Yasumi on Twitter. The chapter numbers aren't important as most of the comics are unrelated to the rest, though some feature recurring characters.