Make a straight cut across the outside came and put that trimmed end into the corner and along the vertical wood strip. The lead should be longer than the leading cartoon to accommodate the length of the upper horizontal. So the length must be longer than the width of the perimeter cames. If it is even longer, the extra can be trimmed off after soldering.
Next butt a trimmed piece of perimeter came along the horizontal wood strip. This one should be shorter than the cartoon. It should be half the width of the perimeter cames to allow the vertical came to butt against it. The reason for having the vertical cames running from bottom to top is that there is a fraction more strength in the heart of the came going all the way to the bottom of the panel, rather than resting on the flanges of the came.
This is how the finished perimeter cames will appear:
These perimeter cames should be held in place with horseshoe nails. Try placing the nails only where a lead line will be soldered in order to cover any nicks the nails might make. Alternatively, you can place the nails at the ends of the perimeter cames to keep them from sliding vertically or horizontally.