I've moved into a new house, and at the bottom of the garden I've found the following. Firstly, a tall wire 'bin' with a bin bag inside, and sticks/twigs inside the bin bag. Then, one quite large wooden 'pen' almost entirely filled with compacted grass cuttings, covered over with a piece of carpet. Next to this is a similarly sized wooden 'pen' with what looks like 'compost' in, then a metal square 'pen' with the same sort of stuff in, with a metal flap at the bottom. I'm really keen to start composting, but I really don't know what this arrangement is for, so thought I'd ask anyone in the know - can you point me in the right direction?