how do i wash oven properly?

Not daft at all. Cheapest method that actually works: Racks out first. Make a paste of bicarb and a bit of water, spread it over the inside (not the heating element), and leave it overnight. Next day wipe it off with a damp cloth and a plastic scraper, then spray a little vinegar on whatever's left and it'll fizz up and wipe away. Racks go in the bath with hot water and a couple of dishwasher tablets for a few hours, then scrub. If yours has a self-clean setting you can use that instead, but take the racks out, open a window, and keep pets (birds especially) out of the kitchen while it runs.