The laser code for the TGP and/or Weapon only needs to be adjusted if JTAC or any other buddy-lasing guy is lasing your target with an laser code other than 1688.
This is because LaserCode 1688 is the default LaserCode and hence already preset.
If if your Buddy (i.e.: JTAC) lases your target with LC 1677 then you'd have to adjust the TGP's and the weapon's (if LGB) LaserCode to the LaserCode provided by your "buddy".
After asserting that your are on the correct LC, put your TGP into Laser Serch Mode "LSS" and it is searching for the laserpoint with LC 1677. Once the TGP found it, TGP locks on to it.
Then you can do a normal LGB bombrun procedure (you may be required to go in hot from the direction where your buddy is lasing the target from, if he is neither airborne nor on higher grounds), and if your LGB is also set to the new LC1677 it will do corrective controls to hit the laser point after "release".