Here is the basic one that I always use.
ch 6 and join with sl st to form a ring.

Rd 1: Ch 3, 2 dc in ring, ch 2 (3dc, ch 2) 3 times, sl st in top of beg st.

Rd2: Sl st in 1st ch sp, ch3, (2dc, ch 2, 3 dc) in same sp to form corner, (ch 1, 3dc, ch2, 3dc in next 2 ch sp) 3 times for 3 more corners, sl st in top of beg ch.

Rd 3: Sl st in 1st ch sp, ch3, (2 dc, ch2, 3 dc) in same sp, * (ch 1, 3dc) in each 1 ch sp, along the side, ( ch 1, 3 dc, ch 2, 3 dc) in each 2 ch sp to form corner, rep from *, sl st in top of beg ch.

Repeat rnd 3 for desired size.