Not much to go on in your question: age group and level of students; context; resource levels; classroom management. You need to start with your objectives first rather than choosing a methodology. What do you want to achieve? How will students be assessed? Will the students be expected to undertake independent research?

Project-based learning is a feature of many primary schools in the UK for example, but is tied to the requirements in the National Curriculum.

It what you mean is that you want to take a thematic approach then how will you choose the theme? Will you as a teacher be working independently or will you enlist the support of colleagues?

Here's a quick example from actual work in the UK. The theme is "People who help us". You start with a 'spider diagram' on the board with the theme written in the circle in the middle. You engage students by getting them to think about people in their neighbourhood. The information you elicit from students is then written on the board as the 'legs' of the spider. Young children will always start by mentioning a parent or grandparent but you want as an example doctors, nurses, postal workers, street cleaners, police officers, school crossing patrol officers etc.

Before you start all of this you will need to work out an assessment scheme - how many weeks, testing schedule etc?

Focus on the lexical groups of vocabulary that go with each job. How will you make the topic/ theme interesting/ engaging to the students? Resources will be important here.

I've seen examples of really bad project-based learning in a Chinese university. One example, undertaken by a young and inexperienced American teacher was where the theme was to make a video. Yes, as vague as that! A few students of this teacher came to me privately and asked for guidance. I asked whether it was a language assessment or a test of videography skills only to be met with indifference when I spoke to their "teacher". Of course one could make this kind of project very useful -scripting, intonation, pronunciation, presentation etc but we have to be clear of the primary objective.