It really depends on your recipe, your additives, how much stick blending you do or don't do, and even your additives.
With a slow-moving recipe brought only to emulsion prior to separating the batter, it's really very easy. In that case, you don't even have to separte the lye solution into portions as Jersey Girl mentions (although I have done that, too with some soaps - it just takes so much more work).
With a slow-moving recipe combined with additives that cause acceleration, I depend on the additives to help with the process. Some examples of additives that cause acceleration or a quicker set-up time: Some fragrances, some colorants.
Say I want to create a design with 4 colors and I know the colorants I have chosen thicken the soap (TD, some greens, some clays) AND my chosen fragrance accelerates. After dividing the batter into 4 equal parts, I divide the fragrance into 4 equal parts, to be added one portion at a time, just prior to pouring that one layer. Same with the colorants, I don't add them until I am ready to pour that particular portion. I'd do the color BEFORE the fragrance, IF I am using an accelerating fragrance.
So first I prep the first layer: mix in the colorant (be it mica or natural), stir by hand or mini-mixer, then the fragrance, stir by hand or mini-mixer. Bring it to light-medium or medium trace & pour. I COVER the mold with cardboard to hold the heat. I wait for it to set-up. (I have even put the covered mold into a pre-heated oven when set-up was too slow.)
Second, I prep the second layer, same as the first. Repeat the process.
This works very well, even for a chevron tilted layer design.
