When cutting for copper foil projects it is important to leave a small gap between the pieces. This is both to allow for the thickness of the foil and for the solder to form a bridge to the other side of the panel.
I prepare the space in the cutting process by scoring at the edge of the pencil thin cartoon line. This leaves the thickness of the line between the pieces. I do not use pattern pieces unless I am using very dense opalescent glass. When I do use pattern pieces I cut them out with a scalpel knife so there is no space between the pattern pieces. I then draw around the pattern pieces and cut to the inside edge of the drawn lines. This also gives a margin for the foil.
Fitting the pieces is still required though. Some people foil one piece and then fit the next to it before foiling it. This is probably the most accurate way of getting a close fit. However, I save up my cut pieces which I have fitted to the cartoon and foil them while watching TV. My cutting and fitting is normally accurate enough that I do not have to take the foil off pieces, grind and re-foil.
While doing this fitting you need to be aware that an additional small gap between pieces is required to allow the solder to join both the front and back of the panel. This forms an “I” beam for strength. The adhesive of the foil is not enough to make a lasting and strong panel. The solder joining the two sides will give the panel the strength to last for a long time.