They are pretty intuitive - go to the edit dropdown in the top right and change to style, then click on the textbox or other widget, then long description. Change something obvious like the text colour to get a feel for it.
Use the theme editor if you want to apply the same look and feel across more than one form and then override it on a form by form basis with inline styles if you need to.
I added the Long description and help icon, but our issue is help text is coming after the field. We are looking as a popup as I shown in my previous screen shot. Just looking if there is any easy way to implement that solution.