Use thread size or yarn weight of your choice (with hook size to correspond). There's a pattern for 4 rows below, with instructions on how to crochet more rows. Stop whenever you have the size half circle you are looking for.
ch = chain, dc = double crochet
Repeats in (parenthesis) explained: http://crochet.about.com/library/blrepeats2.htm
To start: chain 4, make 5 double crochet in the 4th chain from the hook.
Row 2: ch 3 (counts as first dc), turn, dc in first dc, 2 dc in ea of the next 5 dc. (12 dc in this row)
Row 3: ch 3 (counts as first dc), turn, 2 dc in next dc, (dc in next dc, 2 dc in next dc) 5 times. (18 dc in this row)
Row 4: ch 3 (counts as first dc), turn, dc in next dc, 2 dc in next dc, (dc in ea of the next 2 dc, 2 dc in next dc) 5 times. (24 dc in this row)
To crochet more rows: On each new row, continue to add one more dc in between increases (an increase is: 2 dc in next dc).
