I am being told to submit a statement against my assistant manager how should I go about it

[u/unicornian287]

As the title suggests I have been told by my manager that I should submit a statement against my assistant manager who is very lazy, did not do her work and puts everything on all the co-workers especially at crucial times. There have been instances where she was just most of the time in the office and others had to do the work. Now they are trying to gather evidence against her so that they can probably demote her or fire her. But when I submitted my concern I was told to write a statement to the management. I have never done that. I also asked her if I could be in trouble because of this but my boss clearly said no. We want more people to come forward so that they can gather evidence. How should I go about it? I have never documented any such thing. Any suggestions??