This is a step-by-step instruction with photos and video tutorial.

Yarn
Cup coaster requires approximately 45-50 grs of cotton yarn (15-17 grs of each colour: red, white and blue).
Weight/length: 50 g = approx 70-75 m.
You can use any cotton yarn with a similar weight/length or thiner (if the yarn is about 50g=130-140m, you can work with a yarn held double, use 2 strands of cotton yarn).

Crochet Hook 4.0 mm (or refer to the yarn label for recommendations).

Tapestry needle.