1. Unless you've been provided with a static IP address by your ISP (very unlikely), then you need to setup Dynamic DNS. In the DDNS option in Control Panel, you can set up a free account with Synology - remember the host name you assign it.
2. You'll need to setup port forwarding and firewall rules on your router to allow the PC to communicate with the NAS across the internet - you may find the Synology can do this for you by going to the Router Configuration option in the Control Panel - this will give you the port you need to open at least for each application you want to access.
3. If you've configured the firewall on the Synology, remember to allow traffic to all Source IPs on the CloudStation port.
4. Once the above is done, the key is then setting up CloudStation to connect to your Diskstation using the host name you set up in point 1 e.g. <something>.synology.me - this means when you're out and about, it will connect to the router's external IP address on that port and the router will then forward the traffic on to your NAS - job done!