Zen mindfulness is principally no mindfulness (wunian 無念). If that is still too obscure then one can always watch a phrase (kanhua 看話), i.e. a word head (huatou 話頭). In case that proves to be difficult for the moment, it is best to be mindful of the Buddha (nianfo 念佛) as much as possible. That can lead to asking 'Who is mindful of the Buddha?' (nianfo shi shei 念佛是誰), and with that may come the realisation of no mind (wuxin 無心).