Decide on a date- There's always one who wants to put them up as soon as the clock strikes twelve on December 1st- but the other has to rein them in and give them a date to look forward to. 'The novelty will only wear off if we put them up this early' is their usual line of choice. They're right, but it still feels like they're deliberately trying to kill your buzz.
Bring them all down from the loft- First you have to figure out which boxes contain said decorations. Then you are faced with boxes upon boxes of muddled, dusty decorations and in that moment you wonder if you can be bothered after all.
Get confused by the tree- Yes, it has colour coding but why do they have to make the colours so damn similar?! By the time you get that done- you feel like giving up.
Argue over where the tree will go- It has to be near the couch so everything is in easy reach on Christmas morning, but not in the way of any doors and near a plug socket. The logistics make your head hurt.
Get tangled in the lights- Who knows what goes on in your loft while you're not looking- you're convinced there may be come gremlins up there whose sole purpose is to knot fairy lights.
Figure out which light has worked its way loose- One, just one bulb, has managed to wriggle out of its holder and you have to tighten each one or the whole thing won't work. Then you discover it's not the bulb after all but your beloved pet that bit through the wire last year.
Only put up some of the decorations- You may have redecorated during the year and no longer match or your tastes have changed so not all the decorations that went up last year will go up again. But one of you likes to make the place look like Santa's thrown up in your living room by putting everything up you own, while the other would rather your decorations are tasteful rather than tatty.
Argue over the decorations- One of you loves cheap crap because 'that's what Christmas is about' (apparently) and the other likes colour coordination and a 'less is more' approach. You're always going to butt heads every year over this difference of opinion.
One of you follows the other around the room- And move everything they've put up to where you want it and adjust tree decorations so they're more appropriately spaced out.
Sit back and enjoy all your hard work- Despite the many, many arguments, once it's all done, you can cuddle up on the couch, drink in hand and take pride in your appalling decorating skills. But it's Christmas so just go with the flow!