Needed:

Recommended premixed Subaru coolant for your model car.

Burping:

Think of it like you burping to release the gasses from a fizzy drink, when you're filling up your radiator fluid sometimes 'most times', you will get air pockets. In order to releases these air pockets you need to get rid of them 'burping'. The way you get rid of them from an engine perspective is by placing your hand around the top radiator hose and applying pressure.

Process:

Step One) Fill coolant up as much as possible through the cap you previously mentioned;
Step Two) Place hand around the top radiator hose and apply pressure 'you will notice the coolant level decrease in the top canister;
Step Three) Repeat the above numerous times till you can't put anymore coolant in the top canister;
Step Four) Start engine and repeat the above process until you cant put anymore coolant in the top canister;
Step Five) Slowly take your car for drive up and down the street 'making sure to keep an eye on the temperature gauge on your dash 'it may spike, stop car if you see your temp gauge move a quarter of the way up from the middle line and repeat step Four;
Step Six) You should be good to go now (you may want to repeat step five a few times, picking up speed every time to ensure you've got the coolant to optimum level.).

Try not to spill any coolant on the engine as when it heats up it will smell like you have a leaky hose 'this will pass'.

Make sure to keep excess coolant in the boot; as mentioned above by other posters you may have airlocks, so you'll need to top up once they're released also keep an eye on your temperature gauge over the next few days.

Optimum needle level on your temperature gauge should be at slightly less then half level.

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