I had a quick look and we'll need to figure out how shairport-sync works in more details. But, strictly speaking the Speaker pHAT is supported through the existing
airdac script although you won't get the VU meter going.
... so, until further notice, well:
curl https://get.pimoroni.com/airdac | bash
is all you need.
If we find a way to support VU then we'll make sure it can be set up from that same script (and will post a notice here).