You’ll need a sharp but flexible knife (something like a butter knife is good).
Place the scallop in your hand with the flat side facing upwards. Insert the knife into the hinge end of the shell and twist to pop it open. Wedge your thumb facing upwards inside the shell to keep it open.
Slice the knife along the inside of the top shell, keeping it as close to the shell as possible, to detach the meat. The shell should pop a little bit open with your scallop meat still attached to the curved side of the shell. Use your hands to open it up fully and break off the flat top shell.