First and foremost they MUST understand that once you've joined you CANNOT leave under ANY circumstances,
Oh do stop being melodramatic, Jabberwok.
There was, in fact, a very well thought out and detailed plan for leaving which had every chance of succeeding. It would have involved incremental steps and taken place over a long period to accommodate the massive changes required to law, regulation and the agencies required. But it was ignored and would in no way have satisfied the uninformed but well manipulated masses who think that leaving 40+ years of legal and regulatory integration can be cut off, pouf, with no detriment to the UK.
However much the 'leaving a club' analogy is pushed it just isn't that simple. There isn't an analogy that really applies but the 'club' one worked wonders at persuading the simple minded that it would all be absolutely fine...
I, and others, are still agog to hear how being in the EU has harmed the people who voted Leave (personal stories, please, not woolly generalisations about 'sovereignty and border control etc.) and what the tangible benefits are going to be.