Anyway, back to the thread. I think that it has to happen on two levels:
That any strong left leave voice was unsurprisingly absent throughout the EU ref when both capital and media interests were invested in remain and right-leave. In fact the most vocal stuff I heard was from the Morning Star, and some statements put out by ASLEF/RMT (in terms of more publicly recognisable 'names'). The latter's voice was rather drowned out by many of the other unions, and the TUC. And yet, there was strong pro-working class and anti-neoliberal ground to get out there - the EUs involvement in forced austerity over Greece, and what's recently been happening with weakening France workers rights. That the 'social chapter' era of the EU is long gone, and its been following an aggressively neoliberal path ever since - its opening up and liberalisation of markets and how that further privatises public services, how that affects workers rights and pushes down wages, etc. So, some sort of left-leave campaign/coalition of groups in that sense would be internet, email based, etc.
However, filling the void of the left in terms of electoral politics (especially Labour) means actually reaching people and going back to doing the hard work on the ground and in communities. The IWCA had success with going back to this approach which parties have largely forgotten until they are desperate for someone's vote at local/general election time. Communicating on the doorstep and possibly through alliances with local campaigns and direct action groups such as those defending social housing, etc. is a way to achieve this too. I've been involved in action in this area over the last few months - whereby not only does it allow to fight for the primary aim of social housing, but also allows to build up 'consciousness' again - empowering disconnected people to stand up against other issues local to them, but to also help in fighting against the infiltration of the right to feed off such situations. It can develop initially 'single issue' politics and local campaigns into wider vehicles.