billboarding a vertex array

i''ve done billobarding with simple quads, but can you give me some hints on how i could billboard the following vertex array ? for (int zvar = 0; zvar < MAP_Z-1; zvar++) { glDrawElements(GL_TRIANGLE_STRIP, MAP_X * 2, GL_UNSIGNED_INT, &g_indexArray[zvar * MAP_X * 2]); }