FlexForm is a parser function. You can use it on any page and in any namespace. By default it will render plain HTML5 forms with no styling. To bring forms to life and make them attractive, you will need to add styling to a FlexForm or render using themes.
FlexForm syntax is much like creating a standard form in HTML5, so if you are familiar with creating HTML5 forms, you should have no problems with FlexForm.
If you are not familiar with HTML Forms, then this link to W3schools might be a good start.
Example:
This is a simple HTML5 form :
<form>
<label for="name">Name</label>
<input type="text" name="name" />
<input type="submit" value="submit" />
</form>
This is the same form, but now build with FlexForm :
<_form action="get">
<_label for="name">Name</_label>
<_input type="text" name="name" />
<_input type="submit" value="submit" />
</_form>