The purpose of this study is to provide a guide to systematic professional training of play therapists in-training by examining what play therapists in-training experience in school play therapy on-the-job training, group supervision and counseling skill training group and how they grow. The group supervision was divided into pre-support and course support program, and the course support program was sub-divided into a group supervision led by the professor and a playtherapy skill training group led by the doctoral students. A focus group interview was utilized, and six play therapists in-training were recruited. Colaizzi phenomenological approach was utilized to gather, analyze and report the findings from the study. The analyses included thirteen themes and four theme clusters. Four theme clusters included: (1) I need more preparation to stand alone. (2) Struggling in the field too different from what I learned. (3) Build a pillar as ‘a therapist’ through group supervision. (4) On-the-job training and group supervision experience become ‘Assets’. This study is significant in that it provided the basis for systematic training of play therapists in-training.