It’s really up to you, as there’s really not a wrong way and you can just do the way you find easier. If it was me, I’d mock up a quick muslin, using some cheap fabric. If you can, put on the muslin, inside out, and then pin the legs to get the pant leg width that you desire. After you pin, make sure you can walk, bend at the knees, and sit down comfortably. (Sometimes, we go a little overboard to get the style we want and forget about ease.) Take off the pants cut off the excess fabric, leaving enough excess for seam allowance. Then take the basting stitches out of your muslin to deconstruct the pants, place them on your pattern (or a copy of your pattern) and alter the pattern. Then you can use the new pattern to make the pants out of your nicer fabric. Hope that helps! 🙂