飲む分だけ豆をローストして、ミルで挽くんです。

、。

I roast just enough beans for what I'll drink, then grind them in a mill.

Literal

Drink amount [only-だけ] beans [object-を] roast-and, mill [with-で] grind [explanatory-んです].

A coffee enthusiast's routine. 飲む分だけ ('just the amount for drinking') uses 分 as 'portion/amount.' ローストする is the loanword verb for roasting. ミルで挽く ('grind in a mill') — 挽く is specifically for grinding beans or grain. The んです ending explains a personal practice, inviting the listener into the speaker's world.