I agree with Ice. Synthropol is more of an option...not a necessity. Regular laundry soap works well too.
With procion dyes, the longer the fabric stays in a Hot water bath, the more time the dye molecules have to penetrate and attach to the cotton fabric. I often leave the fabric in much longer...several hours longer...even overnight...keeping the water as hot as I can for as long as I can. The longer the bath, usually the more vibrant the color I get.
I like to put the fabric into a smaller dye bath container, then put that container into a bus pan size tub filled with with hot water. Much like a double boiler. This way, I can keep adding hot water to the bottom pan without diluting the dye. Rinsing with cold water will help set the dye, and, as Ice said, salt & vinegar will also help.
Another option that some people like to use is Retayne. But again, it's an option...not a necessity.
Welcome to the board! And, good luck.