You only need to establish one vertical and one horizontal came to begin with. The other two will be placed at the conclusion of the leading.
Normally, you will be working from the lower left corner toward the upper right corner of the pattern if you are right-handed. The reverse is the case for left-handed people.
Hold each piece of glass in place with some scrap lead and nail. The scrap lead will prevent the nail from chipping the glass. It's important all glass is held in place with nails so no shifting occurs while working in another area of the panel.
Fitting the rest of the glass to the cartoon is described here.
Revised Feb 2020