How to create a homebrew rule on critical dice damage

[Tycandus]

In the 5e game that I run we use a different method of critical dice damage. On a crit, there is still the two dice rolled, but we use a max damage roll on one die, and then the random roll on the other. This was done to avoid the situation where you have a "critical" strike, and then roll two ones, resulting in this critical strike, that does less damage than most normal strikes. Currently the critical roll in FG is set up to just roll two dice. How could I go about changing that?

[Zacchaeus]

You would need to write an extension. Having a look at how this one is constructed might give you some clues https://www.fantasygrounds.com/forums/showthread.php?34431-New-Extension-TheoGeek-s-Improved-Critical

[Tycandus]

Thanks! that extension actually fits exactly what I was trying to do! :D :D