At a recent book signing in Barcelona, Spanish YouTuber Chusita was approached by two parents with their teenage daughter.Although Chusita’s book is a sex guide for her teenage fans and followers, she sometimes hears from parents who thank her for writing it, and for helping them broach a difficult subject. Some have told her they’ve sat down with their teenage children and read it together. This time, though, the parents wished to thank her for a different reason.

“Loudly, in front of the whole crowd, they said they’d bought the book for their daughter, but decided to read it first to check it was OK,” says Chusita. “They then told everyone how much they loved the book and that it had rekindled their sex life. Their daughter stood beside them, completely mortified!”

In some ways, María Jesús Cama, Chusita’s real name, is similar to the UK vlogging sensation Zoella – although a much less polished version. The apartment she shares with a friend in Madrid is a long way from Zoella’s £1m house in Brighton. When we talk, she wears no makeup and her YouTube channel, Chusita Fashion Fever, is a fun, imperfect mix of unsophisticated pop covers (Adele’s Hello in Spanish anyone?), confessionals, random musings and straight-talking agony aunt-style advice.

Her book This Is Not a Sex Book: The Uncensored Manual for All Things Intimate was published last year in Spain, where it sold well. It has since been published across Latin America, as well as the US, Holland, Germany, Italy, Slovenia, the Czech Republic – the list goes on. Next week it’s published in the UK.

It’s an odd read. In some ways it’s sweet, almost innocent – with cute quizzes, comic-strip storylines, emojis and “fun facts” such as the “world’s largest penis” or the mating habits of seahorses. There are chapters to help teens to navigate today’s complex modern relationships (hook-ups, sexting, friends with benefits, etc). And then there’s the lowdown on actually doing it – masturbation (male, female and mutual), sex toys (real and improvised), sex (vaginal, oral and anal) … Perhaps it’s pitched perfectly at today’s youth, the internet generation, who on one level “know everything”, yet In Real Life (#IRL) may be no more sophisticated than the generations before them.

According to Chusita, it’s aimed at young people aged 14 to 20 (her fanbase) who haven’t had much or any sexual experience, but who want to be fully informed when they do. She based it on the kinds of questions she is asked on her channel – and also what she wished she’d known herself but had to learn by trial and error.

I get loads of questions. Many times, it’ll be ‘my boyfriend wants to have anal sex but I don’t like the idea of it’

Chusita, 30, who dropped out of school, certainly never imagined or planned this career path. She grew up in Madrid, the youngest of six siblings, and was educated in a convent primary school and a secular state secondary.

The vlogging began when she was 21, living with her then boyfriend and his family, and working as a receptionist. “My boyfriend and I weren’t getting on well. Work was busy. Problems piled up and I started to get depressed,” she says. “I went on antidepressants and for seven months lay around feeling low.”

Facebook Twitter Pinterest An illustration from This is Not A Sex Book.

On a night out with a friend, they met a group of boys. There was a bit of sexual tension, and the next day Chusita made her first video – back when few people did – about “sexual tension in clubs”. She sent it to her friend to watch; her friend passed it about. Chusita made more – about the music she liked, what happened in the supermarket, her thoughts on her daily life. “It was a kind of confidence booster, a sort of therapy.” Her following grew.

A popular section of her channel is If I Were You, where fans send in problems and Chusita reads them out and gives advice. It’s not all about sex – but a lot is. “I get loads of different questions, every possible sort,” she says. “Many times, it’ll be ‘my boyfriend is insisting he wants to have sex and I’m not sure’ or ‘my boyfriend wants to have anal sex but I don’t like the idea of it. I think it’s horrible. How do I handle this?’. Often, the questions will revolve around saying no or setting limits.”

Sitting on her bed, giant cuddly toys in the background, Chusita dispenses refreshingly sensible advice on her vlogs. Her repeated message is finding what’s right for you (whatever that is) and not being pushed around. She shares plenty of personal experience in her posts. My First Kiss – aged 16, on the school bus, she remembers “a lot of saliva”. My First Time – she was the last of her friends to have sex, nearly 19, as she preferred to wait until she was with a person she felt comfortable with. Nonetheless, the sex was forgettable.

Other popular If I Were You posts include “Sex with my cousin” (from a teenage boy who was seduced by his cousin and is debating whether to “go the whole way”) and “I can’t orgasm” (an 18-year-old who lost her virginity to her boyfriend four months ago and has yet to orgasm with him – though she can do it fine on her own). Chusita’s advice on the cousin question is to think years ahead, at family events – this cousin will be in his life for ever. Of course, being sexual feels good, she says. That’s normal, but personally, she’d find someone who wasn’t in her family to do it with. On the orgasm problem, she wonders if the couple are trying too hard, focusing on the problem instead of relaxing and letting herself go. She also urges her to tell her boyfriend what she likes, give it time, be patient.

Why does she think people come to her with their questions? “I think it’s partly the fact that people prefer to listen to someone they don’t know,” she says, “someone outside their circle who doesn’t have any preconceptions or ideas about them. Someone with no agenda. I’ll also tell it clearly – there’s no holding back, no taboos. I’ll talk about anything.”

In Catholic Spain, says Chusita, there’s a huge gap in sex education waiting to be filled. “Sex education was non-existent when I was at school and it’s the same now,” she says. “They’ll talk to you about reproduction but not about sex. They’ll talk about how to make babies but not how to avoid making babies but still have sex.”

Nor is it talked about much at home. “The only thing I was told by my parents was ‘Don’t get pregnant’ and ‘If you do get pregnant, you will have the baby’. I talked about sex to friends, of course, but they were as clueless as I was. We could share experiences, but the only way we could really learn was by doing it.”

She doesn’t pretend to be an expert, but that’s her appeal. She’s like a wise big sister

According to Chusita, little has changed since then. “Young people still have a lot of questions – ‘What am I supposed to do? How am I supposed to feel?’ – and no one is covering them. Teachers and parents are too embarrassed to raise it and teenagers are too embarrassed to ask.”

And into this void steps pornography, available everywhere at the tap of a phone. “I don’t have a problem with porn, but it’s no way to learn,” says Chusita. “Watching a porn film is like watching an action movie. You might think, ‘Wow! That looks amazing. I’d love to do that’ but it’s not the reality. You can’t jump from buildings. Porn is not a portrayal of reality so when you start having sex with someone, you shouldn’t think you’re going to experience a porn film.”

She doesn’t pretend to be an expert, or even to know any more than the average woman. But that’s her appeal. She’s like the warm, wise big sister any teen would wish for.

“I’m not a professional and I don’t have very different sexual experiences to anyone else,” she says. “I think probably everyone finds it harder to talk about sex honestly when they’re starting out than when they get older and realise it’s not such a big deal. Probably anyone who has had sex a few times could write the book. It’s just that I’m the one who did.”

This Is Not a Sex Book is, says Chusita, a more careful, more considered account of the advice and information she gives on her channel.

“I had more time to think about it. It’s better conveyed than in the videos,” she says. “It’s the book I’d have loved to have read when I was a teenager, written in young people’s language, in a way that’s accessible to them. I want it to be part of people’s libraries, the book teenagers go to.” And from time to time, their parents too.

• This Is Not a Sex Book by Chusita Fashion Fever (Head of Zeus, £14.99). To order a copy for £12.74, go to bookshop.theguardian.com or call 0330 333 6846. Free UK p&p over £10, online orders only. Phone orders minimum p&p of £1.99.