You can put panels in pants that already fit you, but bet on wearing a belt with them. I recommend panels that are 2-3" wide.
Remember to add 1/2" to the width of your panel for seam allowance.
Length:: Measure your pants along the open leg seam from top to bottom. Now, measure the height of the waistband, which should be just a few inches or less. Add this waist band height to the leg seam measurement. For example, 40" leg seam + 2.5" waistband = 42.5" panel.
Next, we will prepare your panels.