I know that some children really struggle to remember which way around a b and a d go.
Here are a few things to support them to remember.
1. Talk about the formation of the letter. RWI use items to support this.
b - down the boot and around the toe.
d- round the dinosaurs back and up it's neck.
2. Use their hands to make a bed, with thumbs up. It should then look like a bed, with the bedpost for the b first and d second. (If not the post is in the middle!)
Hope this helps.