I have an idea that you might consider if you're wanting to add knitting to the bottom of a shirt or dress.
First, though, here are two common options, which you've already seen: 1) Attach yarn all along the bottom of the garment and knit your design from the edge of the garment down, or 2) use the same cast on number as #1, but knit the insert without attaching it first; then attach the insert later. Both of these methods require you to have exact gauge and lots of precision so that you don't bunch up or stretch your garment along the edge where the insertion is.
My idea is to cast on for the total width of the insert and then knit it as long as you need it.
This would, of course, turn your pattern sideways, but with many patterns that's not an issue and/or it adds an interesting design element.
One benefit of doing this is you can block and measure the insert as you go, and you wouldn't have to really worry much about gauge (except for the width of the strip maybe, but that's not as crucial as the length).