It is important to have the came fit snugly to the glass (assuming the glass to be the right size and shape). If it does not, the panel is likely to grow beyond the intended dimensions.
To ensure the came is tucked snugly against the glass, you use a fid of firm material (wood or plastic, for example) to press against the heart of the lead. You can press directly toward the glass, or make multiple passes along the length of the came to ensure the heart is touching the glass all along its length.
You should avoid steel tools, because you may cut the lead, and if the blade is long you will not find it easy to fit along all of the curves.