How to roll dice for the UA2020 "Piercer" feat

Please forgive me if this is the wrong forum (even better, redirect me to the correct forum) I am trying to figure out the correct command to roll damage for the UA2020 Piercer feat (I have included its text at the end of the post...) As a concrete example, let's say I am rolling 2D12 and want to re-reroll the lower one if it is less than or equal to 6. It would be nice to have the rule re. critical damage, but that is much less important since it will only happen once in twenty times, so I can handle it manually when it happens. Piercer You have achieved a penetrating precision in combat, granting you the following benefits:  • Increase your Strength or Dexterity by 1, to a maximum of 20. • Once per turn, when you hit a creature with an attack that deals piercing damage, you can reroll one of the attack’s damage dice, and you must use the new roll. • When you score a critical hit that deals piercing damage to a creature, you can roll one additional damage die when determining the extra piercing damage the target takes   

You can't really do the second point for multiple dice without the API - you can either automatically reroll both dice if they're under 6, or neither. You can use a "keep lowest" operator to single out the die you want to reroll, but then you've lost your second die roll. The closest you can get to automating it would be to throw a button link into your applicable weapon attacks, for example in the Damage Type area, or in the Description field: Piercing / [Piercer Roll](~bob|piercerReroll) replacing "bob" with your character name. Then create the macro piercerRoll as an Ability macro on your character sheet: &{template:dmg} {{dmg1=Damage change: [[1d12[New roll] - ?{Number to reroll|0}[Old roll]]]}} {{dmg1type=Piercer}} {{mod=[[8]]}} {{dmg1flag=1}} It's a bit clunky, but there's not much else you can do without API. If your weapon does a single die of damage, you can just use 1d12ro<6 to reroll once - though this would be automatic on every roll which may not be ideal. You might want to keep a 6, for example, if you know something is almost dead. The third point seems pretty straightforward, unless I'm misunderstanding it. For a 2d12 weapon you would just enter 3d12 in the crit field.

Thank you very much for your answer. I apologize that I only noticed it now, I thought I would get an email when a reply was posted but I realize now that it is only an email to my roll20 account. I am a newbie at this, so I am not sure I am doing things correctly. I tried the following: I launched the roll20 campaign I play in and then clicked on the "collection" icon on the top far right and chose to add a macro. I gave it a name and pasted the second line of text from your post into the "Actions" section. As far as I can tell it prompts me for a number, and then rolls a d12 and subtracts the number I entered. This seems to be equivalent to using the standard "advanced dice roller" with 1d12 - "number", but without the useful text message.  So the way I would use this is after rolling my damage I would click on the macro, enter the value of the die I wanted to reroll and it would tell me the additional damage. Correct?