I’m glad you asked this question because I’ve been curious about this prospect too. I think to be able to do contract work in the USA, you need to first incorporate your writing business in Canada, then apply for a TN-1 visa. It’s a NAFTA visa for Canadian consultants who are looking to work with American companies but not needed to become a resident of the USA.
As far as incorporation goes, you’ll need to hire an attorney to help you draw up the incorporation and then you’ll probably need to find a different lawyer to help you apply your TN-1. I doubt that it is very difficult to get this accomplished if you’ve got the correct tech writing credentials and you’re not in a rush, but I imagine that you’re going to have to drop a lot of money to get yourself incorporated.