when to cut green manure

Dig it in any time you like. It is all soft at present, digging in will be easy and it will rot quickly. Leave it to flower and set pods and the stems get woody.

Either pull it and compost it or dig it in. To dig it in trample it flat and chop it a bit with a spade then turn it in. Easier if you shred it with a strimmer or lawn mower.