The Submit Post Window
DIAGRAM 4 - THE POSTING PAGE
This is pretty easy to understand and is similar to a basic wordprocessor. You can toggle the view between .html and the text editor depending on which you prefer using the button to the right of the Editing bar (H).
Editing (G)
Before you post make sure you have checked your spelling, grammar, formatting etc. If you have made a mistake you can edit your post for up to 7 days after posting right now, although it should soon be possible to edit indefinintely. All your edits are recorded on the blockchain so people will be able to see if you have altered your text after the fact.
Tags (I)
You add tags to the bottom of your post to make them easier to find. You can use up to 5 tags. The first tag becomes your main tag category. Once set this cannot be changed. The other 4 tags can be edited during the standard 30 day period. If you post contains any graphic content it should be tagged as "NSFW" - failure to do so can result in other users flagging your post. Note there is currently a bug where using NSFW as your primary tag can hide your post - until this is fixed ensure that you DO NOT make this your primary tag. Do not engage in tag spamming - using tags which do not fit your post. This is another way of getting flagged.
Image Credits
It is beyond the scope of this post to discuss the legalities of what kind of images you should use in your posts. There are a number of posts that discuss this. It is a good idea to list your image sources and provide credit. How you do this is up to you. If you are claiming credit for images then it is usually expected that you have some sort of verification for this.
How do I add images to my posts?
Steemit does not store any images on the blockchain itself. Instead it stores links to images. In order to do this those images need to be stored and acessible somewhere else - something called hosting. I can't list all the potential image hosting sites here but some examples include imagesafe.org and Steem.img.
Personally I always use Steemimg.com which is a free service created by one of our very own Steemit users 
Addendum - after a recent update you can now add images directly to your post by dragging and dropping OR you can continue to use the old method of using a hosting site and linking to it.
What is Promotion?
You can pay SBD to have your post (or someone else's) displayed on the promotion page. They are ranked according to the amount paid. It is illustrated in the diagram below as box O. Note as of the time of the latest update (late November 2016) there is a bug with this feature which has lead to it being disabled until it is fixed.
Posting Rewards
When you make a post on Steemit people are able to reward you by up-voting your post. The amount earned depends on the amount of SP they have (it is a little more complicated than that but I will cover that later). The dollar value of your earnings is payed out, half as SD and half as SP. Posts payout after 7 days and comments now payout with their own independent 7 day timing (separate from the parent post). You can also now choose to receive 100% of your payouts in SP (J) or decline payout altogether.
The Reward Pool
Since a fixed amount of Steem is produced every day then the amount available for rewards every day is also fixed. Depending on how many people vote and curate those rewards are then apportioned to each of the posts. Although the value of the reward pool in terms of Steem is fixed, the value in terms of Steem Dollars earned by each post will vary because the SD is equivalent to 1 dollar worth of Steem.
Why Do the Earnings for My Post Go Up/Down?
For a few reasons. People may unvote after voting you up. The Steeem price will vary throughout the day. The actual value of each vote is calculated using a prediction algorithm which may differ from the actual value which results from the daily rewards being divided by the number of votes. When the value of Steem goes up, the rewards in terms of Steem Dollars will increase, when it goes down the opposite will happen.
The actual values on posts are only estimates until the time of the final payout when the actual payout will be formalised.
How do I format text in Markdown?
Most people are familiar with .html but not so much with Markdown. You have to use markdown if you want to format your text in responses. This is pretty easy to learn and pickup. I found the Markdown Cheatsheet here very useful for this purpose.
https://github.com/adam-p/markdown-here/wiki/Markdown-Cheatsheet
What is Verification?
Verfication is a process where you give evidence to show that you are the person that you claim to be. This is to reduce fraud and people impersonating well known figures.
There are a number of ways of doing it. You can post a photo or a video (preferably) which shows you holding up a sheet of paper with your handwritten user name on it. You can also verify by posting a link to your Steemit profile on a website which you are already known for running such as a blog, photography site etc. For more information this post by 
The Blockchain Remembers So Be Sure Before You Hit Post
Whatever you post on Steemit is on the blockchain forever. By forever I mean forever. As long as the blockchain exists then your original text along with whatever changes you made to it will be there. So before you hit POST be sure that you are happy letting your post and the information within it live forever.
Please upvote me