Assume we need a  board shape with 0.25" rounded corners. Belows are steps of designing such a board shape.

  1. Please turn on the snap-to-line mode.
  2. On the attribute layer draw a horizontal 0.25" line and snap the line end to the left top corner. Let's call this line "helper line":
  3. Double click the board line to the right of the helper line and delete the left part:
  4. Snap the board line to the helper line:
  5. Rotate the helper line 90 degrees and do the same for the left board line:
  6. Rotate the helper line 90 degres and change its length to 0.5":
  7. On the board layer draw an arc basing on the herlepr line:
  8. Delete the helper line, set the arc diameter to 0.5", and adjust the arc begin/end positions to line up with the board lines.
  9. Repeat the above steps for the other three corners except use copy/paste and rotate rather than drawing the arcs from scretch.