If you already do a BBS square and want to take part this winter go to "my details and settings" on web portal. It is also possible to select not doing the survey yourself but to have someone else to do it this winter only instead. This will not affect your BBS (spring) allocation which carries on as normal. For those not familiar with BBS methodology; the squares available are preselected at random by the BTO, you can't just choose one that you happen to fancy.