Quick answer
Finish the standard route first. The secret ending is a New Game Plus route that requires all 25 collectibles, Moon.exe, six tapes, and the White Tiger Doll.
Recommended order
Complete the standard campaign before using an ending checklist. The hidden route depends on New Game Plus and on items whose purpose is intentionally obscure during the first run.
What carries into the hidden route
Published guides consistently connect the hidden route to the 24 first-run collectibles, Parachute.exe, Data Diver, the New Game Plus White Tiger Doll, Moon.exe permissions, and six secret tapes. The dedicated page presents that sequence behind an explicit spoiler control.