Pema Karpo, 16th century Tibetan teacher of Vajrayana Buddhism:

Don’t reflect on the past. Don’t imagine the future. Don’t meditate with deliberate conceptual labeling. And don’t regard emptiness as being nothingness. Without evaluating as right or wrong any of the objects of the five senses you perceive, face inward and, leaving your attention in naturalness, remain naturally just like an infant. Don’t let your attention wander for even an instant.

Completely abandoning the thinker and what is thought,
Remain naturally like a child.
If you diligently apply yourself to the words of the guru,
The coemergent will surely dawn.

Tilopa also said:

Don’t reflect, don’t imagine, and don’t evaluate. Don’t meditate, don’t think, rest in naturalness.

As the Abhidharma mentions, this mindfulness means nondistraction: “Mindfulness is not to forget the related object.”

I found this in a book on Dzogchen and Mahamudra meditation, Perfect Clarity. As straight forward an instruction as any. Now if only there were time.