I've never done it with cannabis, but I understand its the same way. With salvia, I take a razor blade, cut below a node at a 45 degree angle, pull off the leaves on the bottom node and put it in a glass of water. With salvia I keep them in a humidity chamber. Im not sure if marijuana needs this or not. Im sure a bit of research will pull up more details but I'll leave that up to you.
oops, almost forgot, it should sprout roots after a few weeks and then you put it in soil.