Font icons are usually placed inside a <i> tag. Ionic has default css styles for the icons for easy use. The most basic example of use:
<i class="icon ion-home"></i>
Ionic has some CSS components where you can use Ionicons as a default which have preset styling. The range class in the item <div> will apply correct styling to both the input and the icons inside it. Here's an example of a range slider.
<div class="item range">
<i class...
