Instructions:

Articulator: back of the tip of the tongue
Point of Articulation: alveolar/postalveolar/palatal area
Manner of Articulation: Flap – The articulator strikes its point of contact directly, as a very brief stop.
Voice: voiced – Vocal folds are close and vibrating.

Occurrence:
Dutch (North Brabant, Northern Netherlands), Elfdalian, Enga, Hausa, Hindi, Nepali, Norwegian (Eastern and central dialects), Portuguese (Some European speakers, Brazilian caipira speakers, some sertanejo speakers), Punjabi, Swedish (some dialects), Urdu.
(Source: https://en.wikipedia.org/wiki/Retroflex_flap#Occurrence)