Set slider property to True
Consult the docs on UILayout.prop,
prop
(data, property, text="", text_ctxt="", translate=True,icon='NONE', expand=False,slider=False,toggle=False,icon_only=False, event=False, full_event=False, emboss=True, index=-1,icon_value=0)
To display the property in the UI set it to True. The default is slider = False
slider (boolean, (optional)) – Use slider widget for numeric values
Sample code
def draw(self, context):
self.layout(context.object, "prop", slider=True)
Make sure to set the minimum and maximum of the property, if unset the default values are +/- the maximum possible value on your system and as a result the slider will sit around the middle and be somewhat meaningless.
Tip: Edit Prefs > Interface > Display and enable "Developer Extras" will enable you to right click over any (most) UI element and view source to open the code behind the UI into the text editor.