You must learn to create first and then criticize if you want to make writing the tool for thinking that it is.

如果你想让写作成为思维的手段(其实它原本就是)，你必须知晓先创造然后再批判的道理。

that it is 修饰tool，it指writing。
the tool 名词（词组）作make的宾语补足语，如：
I would make you king over the earth.我会让你做国王统治世界。
He made her his wife.他娶她为妻。
All work and no play makes Jack a dull boy.光工作没有娱乐使杰克的脑子迟纯。