Lorina is probably best for answering this but in my experience there isn't necessarily a set way of writing. I'm sure that we all have different writing styles, so there may not necessarily be a right way of writing up a daily reflection or observation. There may be a set template to use though depending on the service or what exactly you're writing up. Observations themselves come in many different forms/methods (E.G. learning stories, anecdotal observations, running records, photo observations, ect.) so it would depend on the type of observation. For example a photo observation is a series of photos which show what is happening with a small amount of text to explain what is happening in the photos. That's possibly one of, if not the, easiest type of observation to do. But even samples of the child's work is a part of programming/observing children and their learning as it shows what the child has achieved/created. But it will come down to your service as at mine we mainly do learning stories.
I think this article would be helpful for you to look at as it explains different types of observations & how to do them
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Different Types of Observation Methods
You could always practice and have a go at observing anyone or anything to write one up & see how you go. If you have a pet, a child, or a partner observe them & write up an observation on them to test it out maybe?
I'm not sure on how much of the process you would need to do, but another helpful article is this one:
Observations in Childcare
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