Short answer: a chilled, smoothly-iced cake and a ganache cooled to just-pourable — then test one drip first.
The drip is all about temperature and control. Ice the cake, then chill it well so the surface is firm and cold; the cold is what sets the drip crisp. Make a loose ganache and let it cool tojust pourable — warm enough to move, cool enough to stop. Test a single drip on the edge with a spoon or squeezy bottle: if it races to the board, wait a minute; if it barely moves, warm it slightly. Once it stops about halfway, work around the top edge, then fill in the middle.
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