In Windows, there are different ways to do the same thing. I have picked the way I think most people will find easiest. If you are comfortable doing something a different way, that's fine. This guide is for people who don't know, some of whom have asked me to explain how I do it.
Unless you have changed your default settings, whenever you start a post or PM, you will see the text entry box (picture below) with "Rich Text" already selected.
If you want to practise as you learn, do it in a Personal Message addressed to yourself. You edit PMs in exactly the same way as forum posts. You don't need to actually "send" your PM - just click on "Preview" to see how your edits look. Using a PM for practice is sensible because, if you do accidentally click on "Send", it will come back to you: You won't embarrass yourself or bother anybody else 
To keep things simple, my advice would be to type your complete message in the box first, without worrying about editing anything. Remember to leave a blank line between paragraphs, to break the text into easily readable portions. I recommend putting a single space (tap the space bar once) on lines you want to leave blank, because some browsers will delete the blank line otherwise.
Here's the picture, showing you the controls you'll be using:
OK. Let's say you have typed your message now, and it's nicely laid out with blank lines to separate paragraphs where needed. The first thing to remember is you always need to highight whatever text you want to change. If it isn't highlighted, it won't be changed.
If you don't know how to highlight text, you simply put your mouse-pointer at the start of the text to be highlighted, hold down the left mouse-buitton, and drag your mouse to the end of your selection and then release the buttton. You will see then what is highlighted.
Once you have highlighted some text, you can deal with the editing. Just click your mouse on the control/s you want to apply (eg: bold face, italicise, strikethrough and underline). Note that those controls are "toggle switches", which means the first time you click them, they turn "on": Click them again, and they turn "off". You can use more than one effect at the same time so, for example, you can bold face and italicise the same text while it is highlighted.
To select which font you want to use, which size and which font colour, you use the same basic steps. Those controls are not "toggle switches" though. Instead, they give you options in a drop-down menu when you click the little down-arrows (see picture). Just click the down-arrow and pick from the available choices.
Three litle warnings:
Don't get carried away with your new skills ! Too much editing will make your post a mishmash of styles and colour, which is not easy to read and may even put people off reading your posts. Use the controls wisely, for best overall effect.
Remember that editing takes time ! I type in blue and, because that is now associated with my texts, I do it every time. That can be a problem ! Sometimes, although nobody has replied to a question, by the time I have finished editing and phootling around with my post, three or four other people have already answered the Original Poster ! Since there's no point repeating answers which have already been given, I sometimes end-up deleting the entire post. What a waste of effort, huh ? Just so you know ....
Finally, always check how your text looks after you have posted it ! Sometimes, when you have made your edits and selected a text colour, for example, you will find that the Lithium software had decided to restore part of your text to black ! If that happens to you, you'll need to re-edit your post and send it again.
Have fun 🙂
hg
[Edited to include link to re-edit guide]
