Cut away rotted timber with a hammer and chisel. Let timber dry (this can take weeks, and you must cover if it rains). Drill a honeycomb pattern of 5mm holes into damaged area, holding the drill at an angle.
Saturate the damaged area with penetrating wood gel applied with a rag or paintbrush. Protect your hands with rubber gloves. Let the gel soak into the damaged area and allow to dry thoroughly.
Apply exterior wood filler to the damaged area with a putty knife. Smooth the filler so it is level with the surrounding timber surface. Allow it to dry, then sand, prime and paint.