Here is a great crochet pattern for a slouchy beret or “tam”.  You will need worsted or  ymedium weight yarn and a J hook.  Note: each row is ended by attaching the last stitch to the first ch-3 or ch-2 with a sl st.

Ch 4 (counts as first dc).  Dc in 4th ch from hook.  Dc 10 more times in same stitch (12 dc total.)

Ch 3 (counts as first dc).  Dc in same st. *2 dc in next st.  Repeat from * around.

Ch 3 (counts as first dc).  Dc in same st.  *1 dc in next st, 2 dc in next st.  Repeat from * around.

Ch 3 (counts as first dc).  Dc in same st.  *1 dc in next 2 st, 2 dc in next st.  Repeat from * around.
(Can you see where this is going?)

Ch 3 (counts as first dc).  Dc in same st.  *1 dc in next 3 st, 2 dc in next st.  Repeat from * around.

Ch 3 (counts as first dc).  Dc in same st.  *1 dc in next 4 st, 2 dc in next st.  Repeat from * around.

You can continue this increase for as long as you need to to make the hat wider.  For a person with short dreadlocks that are very thick, you would want to make the hat wide.  You would just continue the pattern (1 dc in next 5 st, 2 dc in next st; 1 dc in next 6 st, 2 dc in next st, and so on, and so on, for as many rounds as you need.)

Once you have decided that the beret is wide enough:

Ch 3 (counts as first dc.)  Dc in every dc around.  Repeat this row five times- more if you want your tam to be long.  A person with long dreadlocks would need a long tam, but not so long that it becomes cumbersome.

Once the beret is long enough:

Ch 2 (counts as first sc).  Sc2tog around.  If you have made your hat very wide, you may have to do this for two rows.  There should be 40-50 stitches when you have finished.

Here is a good video showing how to sc2tog:

After you have reduced the stitch number to betwen 40-50, ch 2 and sc in every st around.  Do this for 5-6 rows.  You can experiment with this part of the hat; using a hdc instead of a sc makes it thicker, or you can use a fpdc/bpdc combo to make ribbing.

Wool is a great choice for slouchy berets in winter, and cotton in summer.  Learn more about yarn and crochet hook combinations here.