What would you like to know specifically?
Well with regards to the border, many countries would want to help us. EU and the US are big trading partners, so I guess in both our interests to have a stable border. I also know that the two newest countries, Kosovo and Montenegro, are being assisted by the EU and the UN to manage borders, among other things. Now I'm not saying our border control would be full-proof, cuz no country has a full proof border, but we can try our best.
We can set up a special border enforcement unit, who can receive on site training from specialists around the world. We will have to set up deportation facilities and regulations. It won't be easy, but it will be do-able. But hey, if South Africans from other parts of the country believe in our cause, I say let them join. The movement is not anti-South African, it's more anti-north governance. The only thing I'm worried about is mass migration (especially of unemployed, non-skilled people), which would put strain on our economy. The GDP/capita as the population is now is great for a new middle-class country, but on the edge I'm afraid of falling into a lower-class country if it's strained too much.
Btw the Western Cape wouldn't be a city-state, since the WC is the size of England and has many other towns in it.