You can either do this in an empty server by yourself, or in a real game.
What must be done is, load up a server with the dod_jagd map,
or if already in your own server use command changelevel dod_jagd
Join Axis/Wehrmacht
Grab a TNT and plant it at an objective at the beginning of round.
Grab another TNT. Wait next to the same objective until around 7 seconds are left in the round.
Plant the TNT and run.
The clock with stop at 0 seconds until the bomb explodes, and when the bomb does explode it will extend the round and grant you your achievement (as long as sv_cheats 0 is set to 0)
Enjoy the front!