For the "one year continuous" part, it is referring to the work experience in the NOC declared. It doesn't have to be one year with one client.
I am a freelance translator and I applied as self-employed.
I cannot say that I have a single client that would meet the definition of assigning me work everyday, 5 days a week, for one year. So, in my work history, I simply declared that I was working as a freelance translator "From year X to Present" and provided 3 reference letters from different long term customers, together with bank statements
Just remember the three elements you need to prove: paid occupation, occupation meeting the NOC requirements, full-time or part-time
The reference letters must be submitted when you fill in your application after receiving your invitation to apply (ITA). Remember that you must provide a translation if they are not in English or French.