If you don't know what markdown is and how to use it on steemit check this tutorials github
or steemit.
I find steemit editor to be hard to use. Sometimes it also happens that for some reason our post will be rejected and not appear in the blockchain. If you have not saved it before, you can be pretty irritated then. Also preview option is not perfect.
Fortunately, we can use a markdown editor like StackEdit
StackEdit offers great side by side preview option.
Also it is possible to save your work and edit it later.
Export functions support many fromats such as Markdown, html, pdf.
When you finish work on your post you, use Viewer option to see your post in the fullscreen mode
And here is the finall efect
StackEdit supports all markdown features so if you have only used the steemit editor so far, editor change will be a great relief .