The first thing you want to do is go to a lobby, once your there in the bottom of the lobby you will see a box with a text describing "select one of the vendors to unlock either:" and usually you will see a weapon, and three tabs, the one you will want to click on is the "EQUPMENT" one, a attachment will be there for you to rank up and unlock, once the vendor is settleded in "EQUIPMENT" your rank will be focused on that tab.

Once you reach the needed rank you will unlock the attachment, So you're gonna want to play a few times to reach the next rank and unlock the attachment, and once you level up the attachment will be unlocked and set in your inventory.

Now not all attachments are for all weapons, but once you unlock an attachment, another one will be availible for you to unlock, and once you have reached the desired rank, done, you have unlocked an attachment and will be available in the in-battle customization menu, "C" by default will be the customization key, and once you entered the customization mode, your gonna want to click on your desided attachment, like i said, not all attachments are avalible for all weapons, so once you unlock the first attachment, another one will be available for you to unlock, and hopefully, it will be compatible with your desired weapon,
As you can see, only the Reflex Sight and Suppresor are available for the R4A1. I hope this helps you.

Now that you have unlocked your weapon attachment, it will be availible in the customization menu and will be attached to your badass weapon and i hope this derpy guide helps you.
NOW THE AWESOME CUSTOMIZATION TRAILER.

LATER C: