You have to write those in HTML, which is the underlying format of any formatted email. Unfortunately I can not help you further with HTML, you have to search on the internet and learn it by yourself.
One alternative though, I to write/format your email in your favorite email client, like outlook, thunderbird, gmail, etc. Then select an option on these clients to view the email html source, copy the resulting html and paste on FormEmailer's settings. In gmail, you'll have to send the email to yourself, then on that arrow that shows more options for the email, pick "Show Original". You'll have to search through there to find only the html message part, and not the headers, etc.
Note that when pasting the html in FormEmailer's settings, you'll have to replace all "#" characters (used as placeholders delimiters in FormEmailer) it might have with two "##". So that FormEmailer does not mix those with actual placeholders. Of course, when it's actually delimiting a placeholder you should leave the one "#".
Hope it helps,