If you have a grigri or reverso/guide device, you can belay off the anchor (LH shot). Otherwise with an ATC belay off harness and redirect rope through a high runner (this gives a lot of extra friction).
It's easy to drop a second with an ATC (RH picture below), so please belay off harness and redirect the rope to the second through a high runner on the belay. This puts the ATC in the same position as it would be when belaying a leader (LH picture below), and maximises friction.
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