I doubt that you can do it on the phone, at least - not easily. I am formatting my posts on my computer in pure html. The first time it was time-consuming, but now I have a couple templates I use for all posts. As far as I know - there are couple steemit dedicated on-line editors https://steemprojects.com/categories/editor/
In the end, you can use any html editor and then copy-paste source into steemit.