Any common rooting medium like coarse perlite, compost and coarse sand, or coarse pumice. Rooting hormone hasten things a lot. Warmish, not hot conditions and strong light, not direct sunshine. Once they are rooted they can be potted on to their final position in the ground. They will flower during the first season of "life" under good conditions. Rooting cuttings is the standard commercial methods for the millions of dahlias in the trade. Apical or axilar growth can also be used for cuttings, but those sprouting from the base of the stem in spring seems to be most robust.