In reply to GrahamD:
Generalisation as will vary with lots of things, but stress is the problem/enemy whilst indoors.
Buy true pot grown (some are sold in pots but weren’t pot grown - avoid). Buy early to get it used to your garden/weather first ideally in the most favourable position in the lead up to taking in. Pot grown should be kept in pot, but repotted annual or as necessary with growth. Be aware though Christmas trees don’t naturally like being potted so will only survive maybe five years if treated well, though they could then be planted in garden and may do well there for many more years. Pick your species/cultivar carefully for what you want.
Minimal time in house; forget the twelve days of Christmas and think Christmas Eve for 2/3 days max if at all possible esp if room temp high; and a week to 10 days if you must in cooler rooms. Put in as cool a room as possible, avoid rapid temp changes and avoid high temps so never near radiators, fires, wood burners. If you want to keep in house for two weeks say, you’ll have to accept it is unlikely to survive to long. It will have been preparing for winter and it will be shocked by the changes so weaker when put outside.
Bottom water, not top, cover top of pot with towel etc to minimise the top drying too quick c/w the rest of the pot. Water “well” (as in saucer/container) needs to be checked daily or more topping up as necessary, but avoid too much of root ball being below the water level. Make sure the pot is well watered before bringing into the house (drying winds, low humidity outside could have dried root ball a lot). Any period, even very short, of under or over watering could cause premature death.
Reasonably light location, keep humidity up if possible around and near the tree. Avoid household chemicals/sprays in room as much as possible near tree.
Avoid putting tree straight back out from room temp to frost, general cold, wind, etc. as that will stress it also. Acclimatise again - use fleece or other protection, put in greenhouse, around the side out of wind, etc for a period of time and more so if the weather is worse than it was before it was taken indoors.
Good luck.