You should start by finding out first what would be useful for you mum to know.
This is called Need Analysis. Once you know what your mum wants or need to learn in English, basically how and where she is going to use the language then you can narrow down your teaching material to make it relevant to her needs.
This ICALwiki article on Needs Analysis
explains it in more detail.
Check also the ESL Activities
section for useful teaching tips and ideas.