If you choose to use RealVNC like I'am, you'll need to get 2 things...
1. The app on your iPhone
2. The app on your HTPC
Launch the app on the PC and let the built in server run in the background
Open the viewer app on your iphone while connected to the same network as the HTPC
Connect using the HTPC's IP address and whatever password you set in the VNC setup
To get more advanced...
You can open a port on your router (ie 5933), set a port forward of the IP of your HTPC to your open port.
On the VNC server settings, set the server to listen on that port you opened.
Use a whatsmyip .com website to find your routers external IP.
With your iPhone on 3G/LTE, open the VNC app
Connect to the HTPC from your carriers network by using the routers external IP and your port (ex. 66.208.45.234:5933 )
Now you can access it from anywhere that your cell has signal.
