PhD and manuscript writing in 4 months (3 months full time work on a project)

Hello! I am currently in the last bit of my PhD. I am working on the last bit of my results right now and then I will have to write up the manuscript and PhD thesis. I have 1 month just for writing and 3 months of writing while working on another project. I am allowed to write in the downtime of that project.

My question is has anyone been in similar situation? And what are your advice to get through this while maintaining my head at a normal level.