Changing oil and filter:
It is good practice to check the engine oil before hand so you know if it's using oil or not. Simply done by running the engine for 5-10 seconds until the oil light in the cluster turns off. Stop the engine, wait 30 seconds to 1 minute and check the dipstick. If the engine is cold the level should be at the second to top hole, if the engine is hot the level should be at the top hole (this hole is on the edge of the dipstick before the twist).
If its low be sure to check it regularly and consult your mechanic with your observations.
To drain the oil jack up the front of the car and remove the undertray, it's held in by 2 plastic clips and 3x 12mm bolts. The clips are in the front wheel arches, and the bolts are under the car. Using a 17mm socket or spanner undo the sump drain plug, allow the oil to drain, remove the crush washer from the plug and fit a new crush washer. Whilst it drains undo the oil filter which is on the drivers side. Undo the filter and allow it to drain. Before installing a new filter put some engine oil or a light smear of grease on the seal of the filter, and with a rag wipe the excess oil from the mounting surface on the engine. Screw on the new filter tightly. Wipe excess oil from the sump drain hole and refit the sump plug, ensure the washer crushes when doing it up. Refit the under tray and lower the car. Top up the engine oil with 6.5 litres of your preferred oil. Refit the cap and start the car until the oil level light goes out. Turn off the engine, wait 30sec-1min and check the engine oil. Top up accordingly.
EDIT: Stupid me, H6 oil filters aren't surrounded by exhaust, so you'll be fine! And its 6.5 litres of oil not 4.5 :S haha!