I've just done my CELTA and the way my assignment was set up was to set a piece of written homework. The homework was then marked and I had to set remedial exercises based on any weaknesses in the work.
The student I had was Polish and the problem was articles, so it was a case of finding a suitable exercise in a course book or the internet and adapting it appropriately.
Every CELTA course centre sets up their own assignments so without knowing exactly what your task is.
There is a book we had in our course library, "Learner English" by Swan, which might be of help but I don't remember if Ndebele was specifically mentioned but it might be\worth a look.
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