Pick a physical source and a target. Chapter 4's bits, this chapter's ledger: rolls needed = target ÷ bits-per-roll, rounded up.
Each press rolls the source selected above for real, using crypto.getRandomValues() — your machine's Chapter-6 well. Bits accumulate below. For the dice sources the final trim is a hair off uniform — five or six preimages per output; the clean fix is Chapter 11's machinery.