As a left hander the biggest problems I had were:
As your hand moves along it obscures the text you just wrote.
You tend to smudge it. This is less of an issue for a pencil, but once you start using ink of any kind it becomes a major problem (I was required to use a fountain pen, so my life was filled with smudges and blots).
These two problems can only be resolved by changing the way you hold the pencil. You either have to twist the hand below the line or curve the entire hand above the line so that the pencil writes from above. Your wrist still goes over the new writing, but lifted slightly and with a gap so you can see what you have just written.
Also, left handed writing means that the pen or pencil is being pushed across the paper much more than for right handers, which means it tends to dig in and scratch or tear the paper more. You can buy specialist pens which are easier for left handers to use.
Scissors are also an issue as most scissors are meant for right handed use. If you hold them in the left hand you have the following problems:
The cutting line is obscured by the top blade.
The left-handed cutting action tends to pull the blades apart rather than together, so the paper just flips up between the blades rather than being cut. This is a particular issue with the cheap and safe scissors given to children in infant and primary school.
The moulded grips just don't fit the left hand.
(At my school it was often "We don't have any left-handed scissors today, so you'll just have to use right-handed". It got to the point where I didn't know which hand to use either way. These days I use scissors right-handed).
You've asked for specific advice for teaching writing. I think the ones I would suggest are:
Read up on aids for lefties. Special pens and scissors are particularly important.
Be patient. Writing left-handed is harder than right-handed so it takes longer to master. The unnatural hand positions required also make it more tiring.
Talk to the school. If there are obstacles then work with the school to overcome them.
Invest in a typing course when your child is 8 years old or so. I know this sounds like a sarcastic "let them eat cake" answer, but my son and I (we are both lefties) found keyboards to be a liberation after years of fighting with pens and pencils.