It isn't hard to do but it takes some equipment.
You need: plastisol $40/gallon, colorant $3-5 each color, glitter $3-5 each color and size, softner, hardner, heat stabilizer, good gloves, respirator,a mold $ 50-150 depending on whether or not it is injection, a heat source $20-100 depending on what you use hot plate or microwave, containers $5-20 each depending on your heat source (pans, pyrex cups or silicone cups) and some type of powdered salt.
You can probably buy a lot more than you will use for less money that it takes to get set up. If you aren't going to make a bunch of plastics it will never be economically sound for you to make your own.