The purposes of this investigation were (1) to examine the findings of effectiveness of the process of learning-by-doing; (2) to develop students' skill of designing English teaching materials and teaching English language; and (3) to determine an efficient format of learning-by-doing used for training student-teachers in the skill of teaching English. The subjects of the study were 5 graduate students majoring in linguistics at Srinakharinwirot University in Bangkok, Thailand. This investigation employed qualitative research. The research instrument was a questionnaire designed to garner students' opinions towards learning of constructing English teaching materials and teaching English language. Qualitative data from group brainstorming were taken into account. The results showed that the students benefited from the process of learning-by-doing. It assisted them to discover knowledge of designing English teaching materials and English teaching skills by themselves. It was also determined that an efficient format for training student-teachers should include with group brainstorming at every step. This begins with the principles of the audio-lingual method, cognitive code learning theory and communicative language teaching approaches. Brainstorming was also conducted while adapting teaching materials, during practical teaching, self reflection and peer reflection.