This photo shows before the decoder is installed. The two copper jumpers on the front edge of the circuit board must be removed. One jumper is on the right edge between pins 1 and 8, the other jumper is the middle between pins 4 and 6. After removing the jumpers, tin all 8 pins of the board (apply solder to them).
Remove these yellow capacitors by clipping the leads near the motor brushes. Do not attempt to unsolder them. There is no need to remove the two green inductors with the silver, gold, violet, yellow bands. Remove two jumpers.