Most crocheters have worked in the round and know the "standard" formula for increasing to make a circle: you add the same number of stitches in each round. You might start with a round of eight stitches, for instance, and in the second round you put two stitches in each stitch, in the third round two stitches in every second stitch, in the fourth round, two stitches in every third stitch, etc. This bag follows a similar plan, but some variation was necessary, for two reasons: Firstly, it's a semicircle, and secondly, we have a decorative pattern that calls for special stitches in some rows, which effects how we count stitches and how the increases are placed.
To make a semicircle at this gauge, I figured out after much experimentation, I would start with 4 stitches and increase 4 stitches on each round. Basically, this pattern follows the standard formula -- adding one more stitch between increases on each row, except for the rows where there are special stitches. But, as I experimented with the design, I saw that if I followed that method in this half circle shape, it wouldn't come out symmetrical: wherever your increase points are, you get little "points" in the circle shape, and they would end up in different places on the right and left sides of the semi-circle -- no good! So I had to figure out a solution for that.
What I did was make a center stitch in each row, and the increases are mirrored on each half of the bag. For example, in row 4 of this pattern, you need an increase every 3 stitches. The pattern has hdc, hdc, 2 hdc, hdc, hdc, 2 hdc, then a center stitch (also an hdc), then on the second half of the row you have 2 hdc, hdc, hdc, 2 hdc, hdc, hdc. Up to the halfway point, the increases come at the end of the group, after the center stitch they come at the beginning of the group.
That same method is used on every row of the bag. I think if you take note of this, it will make stitch counting much easier. You might want to place marker in that center stitch and check that your rows look symmetrical on each side of the marker. Keep in mind that the ch 2 that starts each row is counted as the first stitch (as noted in row 2 of the pattern).
On the rows with Bobbles (Clusters), I followed the same plan. The Clusters count as one stitch in the row. Let's look at row 5 closely and you'll see what I mean. In row 5 the increases happen every 4th stitch.
Row 5 (Cluster Row): Ch 2, turn, sk first st, hdc in next 2 sts, (5-dc Cl, hdc) in next st, hdc in next 2 sts, 5-dc Cl in next st, 2 hdc in next st, hdc in next st, 2 hdc in next st, 5-dc Cl in next st, hdc in next 2 sts, (hdc, 5-dc Cl) in next st, hdc in next 2 sts, hdc in top of beg ch – 21 sts at the end of this row.
Ch 2 at the beginning is one stitch, then there are two more hdc, and then we need an increase, because we are at the 4th stitch. Since that's where the first Cluster goes, we need a Cluster and an hdc in that spot. We work hdcs in the next 2 sts, a Cluster in the next stitch, and now we're at the 4th stitch again, so we work 2 hdc. The next hdc marks the center of the row, and after that we do everything in reverse: 2 hdcs, then a Cluster, then hdc in next two stitches -- that's our first group of 4. In the next group, hdc and cluster in the same spot and 2 more hdcs for the next group of 4.
In this way, the two halves -- right and left - are mirrors of each other. Is it starting to make sense?
In the rows with post stitches, I departed from the grand plan described here. In row 6 for example, I knew we would need 8 post stitches to go around the 4 Bobbles. So, when making 4 of them, we also work into the hdc behind the Post, which makes 4 increases in the row. On the other 4 Post stitches in the same row, we skip the hdc behind the post.
Now, I know this leads to some long lines of instruction, but perhaps understanding the designer's "logic" can make it easier to follow.