"prepare properly" is correct and usable in written English. It can be used to refer to the process of planning, organizing, and gathering materials in advance of a task or event. For example: Before the debate, it is important to prepare properly by researching your opponent's arguments and rehearsing your own.
You've got to prepare properly is my attitude.
Because it takes months to prepare properly for a disaster.
"We can't start to prepare properly until we know that," he said.
I was furious at myself for not having taken the time to prepare properly.
"All she wants is a chance to be healthy and prepare properly," Lough said.
They become fatalistic, attribute the outcome to chance and neglect to prepare properly.
Littlewood picked up influences like a scholarly jackdaw, insisting that her company prepare properly.
Being a terminologist, I care about word choice. Ludwig simply helps me pick the best words for any translation. Five stars!
Maria Pia Montoro
Terminologist and Q/A Analyst @ Translation Centre for the Bodies of the European Union