Instructions
Select 'Multiplayer' on the main menu screen of MW2. On the Multiplayer menu, select 'Split Screen' and have two controllers available: one to control the character and one as a "ghost" player.
Select the level "Rust," as it is the smallest level available on which to practice. Choose "Unlimited" for the amount of time during the contest, and select the weapons for both players. Assign the throwing knife to the player instead of various grenades.
Begin the level by finding the "ghost" second player. Because it will never fire back or respond, unlimited amounts of practice can be achieved against the target with the throwing knife. Practice throws from long distances, short distances and throwing on the run.
Attack repeatedly; once the ghost player dies, it will reappear at various points on the map. Repeat the process until mastery of the throwing knife is achieved.