Ideally the process would be as follows (although not very realistic).
Firstly, propagate the idea among the people of the Cape. Voters will be encouraged to put pressure on their political parties to come out in support of Cape independence.
When a sufficient number of political parties support independence, a constitutional conference can be held to draw up a democratic constitution for the Cape. Simultaneously, an approach can be made to the South African government to negotiate independence.
Once a constitution has been agreed it can be put to a referendum.
More realistically the ANC is likely to say NO. Given the current deteriorating circumstances there is likely to be serious turmoil in the country. In such circumstances it would be possible for the Western Cape to unilaterally secede and thereby remove the Western Cape from the turmoil. It increasingly seems that this is the most realistic scenario.