Arrow

First, put the <Float.Arrow> inside the floating element, then add the class:

<Popover>
  <Float>
    ...
    <Popover.Panel>
      <Float.Arrow className="absolute bg-white w-5 h-5 rotate-45 border border-gray-200" />
      Popover & arrow, content...
    </Popover.Panel>
  </Float>
</Popover>

Then add the arrow={5} prop in <Float>, make the arrow keep 5px padding from the float element container, and add offset={15} to keep the arrow away from the reference element:

<Float arrow={5} offset={15}>

But then we will find that the arrow is stacked on top of the floating element, which is not the effect we want, so need to wrap the content below and set the position to relative, and it will move to the top of the arrow. Of course, we also need to set the background color, otherwise will still see the arrow below:

<Popover.Panel className="w-[240px] h-[70px] bg-white border border-gray-200 rounded-md shadow-lg focus:outline-none">
  <Float.Arrow className="absolute bg-white w-5 h-5 rotate-45 border border-gray-200" />

  <div className="relative h-full bg-white p-3 text-rose-500 rounded-md">
    Popover & arrow, content...
  </div>
</Popover.Panel>

Full example of the arrow:

import { Popover } from '@headlessui/react'
import { Float } from '@headlessui-float/react'

export default function ArrowExample() {
  return (
    <Popover>
      <Float
        placement="bottom-start"
        offset={15}
        arrow={5}
      >
        <Popover.Button className="px-5 py-2 bg-rose-50 hover:bg-rose-100 text-rose-500 rounded">
          Button
        </Popover.Button>

        <Popover.Panel className="w-[240px] h-[70px] bg-white border border-gray-200 rounded-md shadow-lg focus:outline-none">
          <Float.Arrow className="absolute bg-white w-5 h-5 rotate-45 border border-gray-200" />

          <div className="relative h-full bg-white p-3 text-rose-500 rounded-md">
            Popover & arrow, content...
          </div>
        </Popover.Panel>
      </Float>
    </Popover>
  )
}