Warm the oil first with little spin. Helps to thin the oil so it runs out easier. Don't try it for the first time outside your front door. It is easy to misjudge how far the hot oil spurts across your hand,when you open the sump plug. I found out the hard way haha.
Try and keep the car level so no oil left in sump.
Loosen the oil filler cap to reduce vacuum,when dropping the oil out. Have a cup of tea to let most of the oil drip out.
Use a new crush washer each time.
Don't overtighten the filter or sump plug as you will damage the threads.
Fill the new filter with fresh oil and rub a little on the seal before fitting. It helps prevent temporary oil starvation and seals better.
Don't dump the waste oil. A garage may take it or take to a recycling centre. There are other uses for it too but I won't say what they are here.
Make sure you have enough oil. I think 4.5lt from memory.
Warm engine after stop and check level again. Check underneath for leaks from filter and sump plug.
Don't use cheap oil or mineral oil. I think they are for running in rebuilt or old classic motors. Apparently mineral oil slows leaks down on old engines due to longer molecules or something.
Use a genuine subaru filter. Lot of expensive and worse ones out there. 10 Euro a filter from main dealer has got to be a good investment.
Use axel stands for easy access. Don't rely solely on a jack. This is the most important safety step. Enjoy.