bear with me...
and if you also want to stand as a Labour and Cooperative Candidate (the Labour and Co-operative Party have a pact between them) then you should :-
...be a member of the CO-OP PARTY (obviously) before the 31 October 2017.
and you will also have to persuade, if you are standing in a "multi member ward" next year, that all your fellow Labour candidates in that ward, want to be a Labour and Co-op candidate and are members of the Co-op (both parties by 31.10.17).
You also have to be a member of an actual Co-op to be a member of the Co-operative Party such as a retail or saving co-op and then get the formal endorsement of the Party. You are also strongly advised to attend Co-op party training events.
Job done. Then get yourself actually elected at the next elections.