One trick of the trade is to have students not lift their pencil up when writing either of the two given letters—instead keeping it on the paper after they write the vertical line as they then draw the circle to the right for ‘b’ or to the left for ‘d’. Then when writing the other of the two given letters, students could be taught to lift their pencil up after drawing the vertical line, then putting their pencil back on the paper to draw the circle to the right for ‘b’ and to the left for ‘d’ accordingly.
I hope this helps! Good luck on teaching Kindergarteners, 1st and 2nd Graders the art of proper handwriting! Believe me, I know what it is like from an instructional angle. 🙂