Two weeks out: measure and order
Measure the main door frame, the balcony railing and any wall you intend to fill. Write the numbers down before you look at any listing — it is much easier to reject a garland that is fifty centimetres too short when the number is already on paper.
Order at this point, not later. Our door-to-door window is 3-6 business days to a metro address and 5-10 business days elsewhere, and in the fortnight before Diwali every courier in the country runs at capacity, so add a few days on top of that rather than assuming the usual speed.
One week out: dry-run the fixings
Put the adhesive hooks up now, before anything is hanging on them, and leave them a full day to cure. This is also the moment to check that the balcony railing clips fit — railing profiles vary far more than people expect.
Three days out: hang the heavy things
Backdrops, arches and lantern strings first, because they are the ones that need adjusting. Anything that hangs from a door goes up last, since every trip in and out knocks it.
The day itself: keep flame clear of fabric
Treat every piece of decoration as flammable. Keep diyas and candles below and clear of fabric garlands, paper lanterns and buntings, and never drape a garland over a light fitting. If you want light in the decoration itself, use a battery string rated for indoor use.
After: store it so it survives the year
Coil garlands loosely into separate bags, roll backdrops rather than folding them, and keep everything dry. Most decoration that gets thrown away after one season was not worn out — it was crushed in storage.