The way Mindfulness was taught by the Buddha had little to do with achieving something. It was more the experience of being present. Passive more than active.
Like being in the passenger seat rather than being the driver.
It was a way of relaxing, settling back and gently establishing yourself in awareness.
Focusing too hard on the instructions can therefore take us away from this quality of resting. Attention, knowing are tools. We are not supposed to get tangled in them. The awareness the Buddha talked about is therefore a non-reactive awareness.
Mindfulness is a faculty of the mind. When you practice mindfulness several things are involved , the two main ones being mindfulness itself and clear understanding. The Buddha didn't give precise instructions on doing, he rather used the verb knowing. Mindfulness was intended as a state and the idea is to establish yourself in that state. And you establish yourself in mindfulness through relaxing or through knowing. One of the most common places where the word Mindfulness was used by the Buddha was in Concentration. An awareness which is, again, being rather than doing. Relaxing and opening up allow for awareness to be there and for things to come to awareness. It's about being present rather than being pulled away from what we know. Once a little mindfulness is established, then it is possible to direct our Attention in certain ways. You can both be passive and active, relax and yet be aware and rest in awareness.
Establishing yourself in presence, settling back is the starting point of our further investigation.
So, Open up. Relax. Be. Then your knowing will be precise yet free and untangled.