Count Your Stitches. When working rows of single crochet, it is easy to accidentally gain or lose stitches. Counting each stitch will help you to ensure that your dishcloth turns out square as intended. If your count is off, go back and fix it before crocheting the next row.
Don't Stress About Uneven Sides. Even if you follow the pattern exactly, the sides of your dishcloth might turn out uneven. Don't worry about this. It takes practice to crochet nice neat sides; if you keep practicing, you'll get to the point where your sides are always consistent.
In the meanwhile, this dishcloth's edging is designed to hide imperfections. Assuming you don't gain or lose any stitches, the uneven spots on the sides of the dishcloth will be hidden under your edging.