It`s probably getting a bit late in the season for green manuring now, but basically the idea is that as soon as a particular crop has finished you sow the appropriate plant seeds, then dig in the resulting plants before they can set seed - i.e. sow in late summer and dig in late autumn. This allows the green manure time to break down in the soil before spring planting.
As to what you sow, this depends to a large extent on what crops are to follow. Old favourites were mustard, rape, lupins, field beans, clover etc. However you would, for instance, not want to sow mustard or rape where you intend to follow with brassicas, nor beans where any other legumes are to follow. You should be able to get a full list of available subjects from the RHS site, the HDRA site, or some of the many other organic sites on the internet with details of which crops they may safely precede.
Alternatively, if tim is listening he will no doubt be able to supply you with a list. Personally I just cover the abundant chickweed with black polythene every autumn.
