When you Prestige the gun once, you can put your Clan tag on it. When you Prestige it twice, you can put your emblem on it.
To put the emblem/clan tag on it, go to the weapon in Create-A-Class and Personalize it by pressing Y. This should bring you to the camo menu. Press RB to go to Emblem and Tag, and press A on the tag/emblem. It should light up orange.
And voila! (or wala, as some ignorant people say). Emblem on your gun! :D
EDIT: Ninja'd :(
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