How does Savage Attacker work?

I want to start this off by saying I am brand new to DND and got dropped into a campaign mid-way as a lvl 7 human ranger, so anyone responding might need to word it in very... clear... wordss...

The thing explaining Savage attacker says this:

You’ve trained to deal particularly damaging strikes. Once per turn when you hit a target with a weapon, you can roll the weapon’s damage dice twice and use either roll against the target.

What I want to know is how to roll with Savage Attacker, which might sound a little stupid but it has been driving me mad because I have no idea what the DM manages and what DnD Beyond does. Assuming that its the DM that applies the damage to the creature I'm attacking, do i just click the damage roll thing twice? (っ◞‸◟ c)

I’d double check with your DM to confirm how they want to handle it. But, basically, yes, you could just click the damage button twice and use the higher number. 

Two things to keep in mind are, you can only use it once per turn, and as a level 7 ranger, you get 2 attacks on your turn. So you don’t get to use it on both attacks, only 1. Also it only applies to the weapon’s damage dice, so if you are using hunter’s mark, you do not get to re-roll that die.

Thank you so much :D