It's a very common problem for children to reverse, or print letters backwards and the letters /b/ and /d/ are among the most common reversals. There are things we can do to help them. As with everything, making the learning and practicing multi-sensory is very important. This means having them use all their learning modalities or pathways (visual, auditory, kinesthetic and tactile) as much as possible.
 
This teaching resource has 12 pages of fun and engaging activities that have the children using all their learning modalities simultaneously.  The fun little stories that go with each letter make it memorable and help kids to remember which side of the circle the stick goes on for each letter.  Having the children say the little expression aloud while they trace or print is key in creating mastery.