"Cold Outdoor Pile
Easy to start. Can add material continuously.
Very low maintenance and the resulting compost is very rich in soil organisms.
Takes more than a year to decompose. Some nutrients are leached and can also attract animals."
Intrepreting the system mentioned above, I just started piling up 'grown' waste (old straw, tough vegetable stems and hedge clippings) and 'green' waste (fruit, coffee grounds, cut flowers, grass clippings) in small amounts every week.
It looks I have to be very patient... that'll teach me for being a lazy gardener!