When you have to increase in pattern, you have to imagine the pattern expanding outward. So when you work the increase on the right edge you have to work the stitch that would come before the stitch at the edge.
On the left edge, it's a bit easier because you continue to work in pattern.
If there are yo's though, you should make sure that there's a k2tog to balance it off.
Or, you could take the easy way and work the increases in stockinette. It's under the arm, it's a baby sweater, and you can decide it's part of the pattern.