Panel lines are a great way to add interest to a design. There are countless variations and it is a way to include more than one fabric in your design if you would like. I really struggle to keep a design limited to one fabric, so often add a contrast fabric with panel lines. Panel lines are also a good way to eliminate darts, without losing the fitted shape of the garment.
I have included some examples to get you going, but try some experimentation of your own. Keep in mind the shape of your body when designing panel lines. For example, horizontal lines are best used to highlight the slimmer parts of your body (i.e. the waist) rather than being used across the hips or bust.
The horizontal panel line can be great when wanting to add pleats or gathers. Cut the pattern in two and then make the changes to the lower pattern.
I used a horizontal panel line on the body and sleeve of my "Two Colour Shirt," so I will use that as an example of how to draft panel lines on your pattern.
Take the pattern you are adding a panel line(s) to. Draw in your panel line (s).
If you are having a horizontal line, ensure that it is square to the grainline.
Mark a balance point on the panel line to make it easier when sewing your garment together.
Trace off each of your pattern pieces, leaving enough room around the edge to add seam allowance.
To finish, add seam allowance.