You need to model a few models! A w_model(how the weapon looks like on the ground) a v_model(how it looks in the hand of a monster/other player) and a p_model(how it will look in your own hands,this model also includes the hands that hold the weapon[If you want the player to see them])
To model you'll need a special program, like milkshape. After you built the models then you'll need to code so the weapon will do something and to include it in your mod!
It's quite complicated the part with the coding and the part with the texturing of the models.