Sorry,
It's a little harder to find but I had found it on the Carpentry/Tinkering Board...
1.8. How do I make a potion keg?
To create a potion keg you will need to follow these steps and have at least 75.0 Tinkering skill:
First, you will need an open keg; these can be the kegs from dungeons, gypsy or brigand camps (have to open and empty before using for the potion keg), or can be crafted. To craft these, you will need at least 57.8 Carpentry skill along with the following items:
3 barrel staves (requires 00.0 Carpentry skill and 5 boards to make - can also use the ones recovered from a shipwreck)
1 barrel lid (requires 11.0 Carpentry skill and 4 boards to make)
1 barrel hoop (requires 00.0 Tinkering skill and 5 ingots to make)
To turn the materials into a keg go to the Carpentry menu, select "Containers", then "Keg".
To turn your open keg into a fully functioning potion keg you need the following materials:
The previously made (or looted) open keg
10 empty bottles
1 barrel lid
1 barrel tap (requires 35.0 Tinkering skill and 2 ingots to make)
To turn the materials into a potion keg go to the Tinkers menu, select "Assemblies", then "Potion Keg". If successful, you will be the proud owner of a working potion keg waiting to be filled.