In this phase of writing, you put your ideas on (digital) paper. The focus is still on the content, a bit on structure, but certainly not yet on correct use of language (this is for the final writing phase).
Below are a few helpful tips to help you write down your ideas:
Log book: Maintain a log throughout your project. For example, briefly write at the end of the day the main things you have done, choices you have made, and other things that might be of interest for writing your report.
Freewriting: The idea of this technique is to just write freely about the topic / problem of your report (or a certain sub-topic within it), for a fixed amount of time, between 5 and 15 minutes. Write down everything that comes to mind. This can give you ideas and is also a useful tool to just start writing.
Brainstorming: Like with freewriting, you write for a set amount of time, but this time your aim is to write down everything, every detail you know about the topic. You may be able to use (part of) what you have written as a preliminary outline for your report. Or there may be certain points about which you want to continue freewriting.
Native language: If your native language is not English, it can be helpful to start writing in your mother tongue, and translate it into English later on. This approach can help you get your ideas on paper more easily.