When you are increasing stitches on a patterned garment, perhaps for a sleeve or some waist shaping, you may encounter the instruction "increase in pattern" or something similar. This is so that you won't have something like a big weird unmatching section of stockinette at the side of your sweater -- if your sleeve is worked in a patterned stitch like seed stitch or a lace pattern, you want the whole thing to be in that pattern, even as it gets wider.
It can be a little confusing when you're adding stitches to both sides. The end stitches are easy to figure out, but the beginning stitches can seem a little tricky. You really just want to work the new stitches on the next row as if they were always part of the pattern. In seed stitch, for example, your first 5 rows will look like this if you cast on three and increase 1 stitch at each end of every other row:
Row 1: kfb, p1, kfb
Row 2: p1, k1, p1, k1, p1
Row 3: kfb, p1, k1, p1, kfb
Row 4: p1, k1, p1, k1, p1, k1, p1
Row 5: kfb, p1, k1, p1, k1, p1, kfb
And so on. You need to be able to identify the components of your pattern to determine where those new stitches belong. In the above example, the most important thing to remember about seed stitch is that you are working every stitch the opposite of what it appears to be, so if it looks like a knit you purl it and if it looks like a purl you knit it, and they're alternating in a 1x1 pattern. When increasing in pattern like this I generally find it's easiest to just look for the first recognizable stitch I can and then count out what I should be at starting the row (e.g., in seed stitch if I see a knit stitch three stitches in, I know I should purl that one, so the one next closest to the tip would be a knit, and the then the first stitch will be a purl). When you get to the end, you'll just continue in pattern alternating knit and purl and that will work those new end stitches in correctly.
This strategy can be used with pretty much any stitch pattern, no matter how complicated: identify a stitch that you know where it falls in the pattern, then work backwards from there to determine which stitch you're starting with. You may have to fudge occasionally...let's say your pattern is something like k3, k2tog, yo, p2. If you're increasing in single stitches, at some point the "correct" stitch to begin with will be either half of the k2tog or the yo--neither of which is really feasible. When that happens, just knit (or purl if it looks better) the edge stitch and begin using that stitch in pattern again on the next row.
Increasing in pattern doesn't have to be tough--just take it slow and remember that your goal is to maintain the overall patterning as you work across.