To keep pieces from moving about as you solder them, use pins or nails to keep them in place. The best is to assemble the whole panel and then keep them in place with a frame or lots of nails/pins around the outside. This keeps pieces from moving and also keeps the panel to the original size.
The type of nail or pin will depend on the work board you are using. Softer boards allow push pins of various sorts to be used. Harder boards will need nails.
If you don’t like assembling the whole before soldering, you can confine the pieces you are currently soldering with nails/pins in the same fashion as for the whole panel.
It also helps to do a little tack soldering before the process of running a bead begins. A small amount of solder on the copper foil where pieces join will keep the pieces in exact alignment while you are running a bead.