Sissy content is usually sexualized feminization that includes:
- Normal feminization stuff - hormones, dressing up, etc - but at an accelerated rate.
- Chastity and maybe sextoys.
- Accepting that women having sex is normal, and by becoming a girl/woman the sissy having sex as a girl is normal. Usually stories portray this with a shock factor of someone close to the sissy having sex without them - girlfriend, girl friend, mother, sister, mistress, etc - but this doesn't have to be ntr. And this precludes the sissy having sex as a girl.
- Obtaining an ideally girlish body through extra means, such as yoga and pole dancing, changing one's diet, mastering makeup, normalizing paying for beauty upkeep like manicures, waxing, etc.
A lot of it is normal trans acceptance type stuff but with an added layer of things usually considered sexual. And the sissy is in a submissive role throughout.