Are there any official guidelines of person-centred therapy? I understand that the person-centred therapy is based on humanistic psychology.

Humanistic psychology is sceptical about the use of the scientific method to study human behaviour. Rogers (1965, p. 190) said that: 'it is the dedicated, personal search of a disciplined, open-minded individual which discovers and creates new knowledge ... no refinement of laboratory or statistical method can do this.' 

Regarding this I understand that the research for humanistic is mostly qualitative and not quantitative. 

What would you refer to as evidence/research for person-centred therapy? Is there any official manual for it? 

Rogers, C. R. (1965). Some Thoughts Regarding the Current Philosophy of the Behavioral Sciences. Journal of Humanistic Psychology, 5(2), 182–194. https://doi.org/10.1177/002216786500500207
Humanistic approaches to psychotherapy are wide ranging and not just person-centred therapy.  Some allow the scientific approach into the therapeutic discussion, and some don't.
Person Centred Therapy (PCT) is very different to the Freudian approach to therapy, and how the therapy is provided will be dependent on the form of therapy the therapist works within — integrative or pure person-centred.
Is there any official manual?
There is no &quot;official manual&quot; I know of, but you can gain a lot of knowledge on the person-centred approach by reading Mearns, et al. (2013) which was a core reading material in one of the years of my training in integrative psychotherapy.  There is also a YouTube copy of an old documentary showing Carl Rogers demonstrating PCT.
The difference with PCT compared to other forms of therapy is that therapy is not led by the therapist.  Therapy and the sessions are led purely by the client and what they wish to bring to the session — hence the name Person-Centred Therapy.  From a pure person-centred approach, you are not necessarily completely shunning the scientific approach to therapy.  You are just not pushing the scientific approach to therapy towards the client.

Distrust of experts runs deep among person-centred practitioners.  The person-centred counsellor must learn to wear her expertise as an invisible garment in order to be an effective counsellor (Mearns, et al., 2013).

The Counselling Directory (n.d.) also points out the following:

In some psychotherapeutic approaches, the therapist and their observations are deemed ‘expert’. The person-centred approach moves away from this idea, and instead trusts that we have an innate tendency to find fulfilment in our own personal potentials (self-actualisation). By facilitating this, the approach helps the client to recognise their own capacity for not only self-healing, but personal growth too.

What would you refer to as evidence/research for person-centred therapy?
I haven't found a PDF or other form of access to the article as of yet, but according the the abstract, Rogers (1979) discusses the theory and research behind this approach.
Some may consider this form of therapy to be pseudoscientific as it doesn't follow the Popper classification of science (Popper, 1959 &amp; 1963), and some believe that although Rogers put the approach forward as a different form of therapy, it should be seen as a way of working as a therapist in all therapies — with congruence, empathy and unconditional positive regard.
Interestingly though, in an encounter group run by Carl Rogers, he said there were times when he had to break all his own rules, emphasising that it was more important to be spontaneous and real than to be carefully empathic or relentlessly unconditionally positively regarding – meaning that sometimes he would violate the core principles of his own theory of PCT.  He might direct a person on what they must do or tell them forcibly that what they were doing was wrong (Geraghty, 2016).
References
Counselling Directory (n.d.). Person-centred therapy. Retrieved from: https://www.counselling-directory.org.uk/person-centred-therapy.html
Geraghty, A. (2016). We need to talk about love. Therapy Today 27(1), pp. 20-23.
Mearns, D., Thorne, B. &amp; McLeod, J. (2013). Person-Centred Counselling in Action, 4th Edition (Counselling in Action Series). London: SAGE Publications Ltd.
Popper, K. P. (1959). The Logic of Scientific Discovery. New York: Routledge.
Popper, K. P. (1963). Science as Falsification. In Conjectures and Refutations, New York: Routledge and Keagan Paul, pp. 33-39
Rogers, C. R. (1979). The Foundations of the Person-Centered Approach. Education, 100(2), 98-107.