can someone tell me where this is please

to customize a weapon you can copy the weapon into the homebrew tools and add to it. After you have saved the copy and renamed it you should then be able to add it to your character sheet as long as homebrew content is enabled on the character creation homescreen

What do you mean by where this is? 

Like, how to work on homebrew? Under Collections there's a whole homebrew section.

You can either create what you want, or browse what others have published for homebrew.