Important size information:
Depending on what you want to make I suggest a 1X1 filet crochet in a thin crochet cotton ie. no.10 (ch 31 + 3 to turn which will count as 1st treble (UK)/ 1st double crochet (USA) of the 1st row) for the bookmark or if you want to make a scarf use whatever type of filet you prefer and use a worsted weight/ DK yarn (or thicker yarn)
http://www.nezumiworld.com/crochet_101.htm for filet “how to read charts”

Please Note: The free pdf download is for the standard 2X2 filet crochet. Beware! depending on the yarn thickness and hook size it may turn out too big for a bookmark (a wide large bookmark is handy in large knitting and crochet books) , but would make a bag/ cushion insert, scarf, pillow trim all depending on the yarn thickness and hook size.

Note: Because I have been asked a few times a Shamrock has 3 leaves and a Clover has 4 leaves (ie. Luckily four leaf clover)