Get a bus to Taliwang, then you'll need to hire an ojek or charter a bemo for the run to Kertasari as there is no regular public transport. If you're travelling under your own steam, if coming from Poto Tano, head to Taliwang, then follow the signposting for Kertasari -- it is about a 20-minute ride from Taliwang.
Note if you are riding or driving from Poto Tano, you'll reach a very well signposted turn-off to the right for Kertasari. While this road will eventually get you there, you'll need to bike through farmland and on extensive (and unsignposted) dirt tracks -- we doubt the road would be passable by car. It is faster (and more comfortable) to go via Taliwang.