Always watch children closely when they are writing. They have no natural instinct to form letters correctly, and it's far easier to establish good habits than it is to break bad ones. The problem of reversing b and d can be resolved with little difficulty if the child is taught correct letter formation.
Handwriting practice should never be combined with spelling practice. If you have to perform two difficult tasks at once, you will do neither one well. The only exception to this is teaching individual letter-sounds to complete beginners.
When teaching older pupils with very poor handwriting, you may have to ignore the problem for a while. Teaching children to read fluently and spell accurately are much higher priorities than improving handwriting. When time is limited, you have to make hard choices. And in most cases, there will be a marked improvement in handwriting as spelling ability improves. Once the pupils are spelling most words correctly, they are far more likely to take pride in their work. Besides, when they know how to spell words, they have a lot more attention to devote to forming letters neatly.