The main stumbling block will always be funding. One idea is a tithe. This is a small voluntary subscription from members and supporters. Basically this is how unions were originally built. Tithing means that the money comes from within the group and is totally independent and it gives members a sense of involvement.
Campaign groups can sometimes get money from unions.
However it is important to appeal directly to workers through their branches. Any approach to the bureaucracy would be avoiding the chance to build genuine solidarity and probably doomed to total failure anyway. Usually and unfortunately, this won't raise enough money. For service based groups external funding will have to be sought. This should not be rushed into on a 'grab it where you can' basis.