The pattern looks as though it came from a blog and was written in a way that made sense to the blogger, but does not follow the "rules of writing" that have evolved over the years for yarn companies, crochet magazines, and other crochet publishers.
The author is using the term "2 sc cluster" to mean "2 sc in the same st."
The pattern for creating the egg-shaped body of the fish is fairly standard:
1. Start with a magic circle (so you can pull it tightly closed), double the number of stitches in the second round, increase by 1/2 the number of stitches in the third round, and so on.
2. Since you start with 6 sc in the first round, you sc twice into each stitch for the second round, increasing 6 stitches and producing 12 stitches total.
3. For the third round, put 2 sc into the first st and 1 sc into the second stitch. Then repeat this two-stitch sequence for the next 10 stitches from the previous round. In this round, you again increase 6 stitches, and you have 18 stitches at the end of the round.
4. For the fourth round, put 2 sc into the first st and 1 sc into the second stitch and 1 sc into the third stitch, then repeat the sequence around. In this round, the designer is cautioning you to be sure to line up the 2-sc increases with the first stitch in the 2-sc increases from the previous round. In other words, *don't* deviate and put a 2-sc increase over a stitch with 1 sc from the previous round or in the second stitch of the 2-sc sequence. At the end of the round, you will have increased 6 more stitches and have a total of 24 stitches in the round.
And so the pattern goes for rounds 5 and 6. After that, rounds 7, 9, 11, 13-14, and 16-21 require a sc in each stitch (no increasing), which shapes the oval body. After that, you change to dc stitches.
The basic rounds are the same basic rounds you will use for anything that starts out as a circle, whether it's the nose of an amigurumi animal, a basket, or a hat. The only thing that may change is whether you use single crochet stitches, double crochet stitches, or some other stitch. Well, one other thing that might change is where you put the increase stitches (where you put two stitches into one stitch from the previous round). When you line the increase stitches up, as in this pattern, you will see definite segments form as your project's diameter increases. When you move the increase stitches around and increase over a non-increase stitch from the previous round, you won't see definite segments form. Each method is a choice that produces a different design element.
One thing that might be helpful for you in this project is to mark the first stitch of each round with a safety pin, removable marker, or bit of contrast yarn so you know which is the "first stitch" each time.