If you are doing a 2 color swirl, then pour the amount of resin you are going to use for each color into your 2 mixing cups. Add the amount of MEKp to both, and stir, by hand or by using Grub32's drill method (I recomend the drill). Now comes the hard part, leave the resin alone for 20 minutes, unless you know that it will gel in that time, if so then leave it alone for only 15 minutes. I usually bell/dome cover my mixing cups with qt deli containers to keep the smell down. While you are waiting on the resin to time-out/thicken start preparing your colors to mix into your resin. If you know how many scoops, fingernails, popsicle sticks of pearl-ex to make your color get them ready to mix them in when your timer goes off. Since you are not using any liquid/paste pigments that inhibit or accelerate curing then you should be able to time this fairly well. After thoroughly mixing in your powders(again using Grub32's drill method), check the consistancy. If you aren't getting to the thicker than honey stage(almost molasas) then let them sit and check them every minute or so, it could take a while. When the resins start to get thicker than honey, stir them up real well to re-suspend all the pigment flakes, have your mold ready, take a mixed color cup into each hand and pour together in an overlapping stream into the mold, or pour as streams crossing each other or pour some of the first color then pour some stripes or blobs of the second color, then pour the first color again and repeat.
With PR the biggest part to getting your colors to swirl instead of mixing is to wait as long as possible before pouring it into the mold. Time is your friend until it is your enemy. You have plenty of time in the beginning, so prep everything, because when the resin starts to go it can accelerate like a race car.