Before you start preparing for Deerclops, you will have to have enough tools and resources to survive a standard winter. Skip to the sixth paragraph if you already have the stuff to survive winter.
You should gather enough logs in winter to keep your fire going all day. Making your base near a lumpy evergreen forest is a good idea, as they produce 3 logs per fully grown tree (4 if you dig up the stump) and you don't have to worry about replanting them. This is a short-term solution, however. If you want a renewable source of wood, plant a grove of evergreens near your base, as twiggy trees do not produce much wood and birchnut trees do not drop birchnuts during winter as they are deciduous.
Making a clothing item for insulation is advisable, though not necessary. A beefalo hat or winter hat is good for your first year, although for the next years you may want to consider hunting Koalefants to make a Breezy or Puffy Vest. If playing as Wilson or Webber, do not shave your beard during winter, as it can be used for insulation.
You should start preparing for Deerclops about five days in advance (about day 25). You should have the above materials by then. The first thing you will need is a weapon. For your first year, a ham bat will do. Feed a pig four pieces of monster meat (easily obtainable from hounds or spiders) to turn it into a Werepig. Werepigs have a simpler kiting pattern than pigs and defeating one should prove easy enough if you have a spear and a log suit. Use the drops from the Werepig to make a ham bat. You should at least wait until winter before you do this though, or the ham bat will spoil. If you are playing as Wigfrid and don't have enough time to make a ham bat or can't find any pigs, a Battle Spear will do.
Alternatively, burn some trees to collect charcoal. Using the charcoal, make some boomerangs. Only do this if you have enough silk though. 15 boomerangs will do enough damage to defeat Deerclops, although you may want to craft more boomerangs for practice or as backup. Remember you will need a total of 60 logs to make that many boomerangs, so a luxury axe is advisable if you don't want to be crafting new axes repeatedly. If you have a bird trap, gather some feathers from birds to make darts, as those are a suitable ranged weapon as well. Remember you will need reeds to make all kinds of darts, though, meaning you will have to visit a swamp.
You will need to replenish your health and sanity during the fight, as Deerclops' attacks pack a punch and its insanity aura is one of the worst in the game. Pierogi is a good idea, as the eggs are easily obtainable from Pengulls during winter and the other ingredients can be either foraged for or obtained from traps. Pierogi also regenerates more health than other crock pot foods, and replenishes sanity as well. Even without ponds to fish from, Fishsticks can be made by entering the swamp and sweeping up after Merm and Tentacle battles.
Dragonpie can be used, although you will need a birdcage to produce dragonfruit seeds if you are to produce a lot of it. Dragonpie is an effective health regen and also regenerates sanity. Beware though; as it is not a meat item, Wigfrid cannot eat it.
Taffy and Jerky are both good options to regain lost sanity, however you will need to take down some bees or make a bee box for taffy, and remember that bee boxes do not make honey during winter. You will need to prepare jerky in advance, as it takes quite some time to dry.
Some characters will not have to prepare this way, as Wigfrid can regain lost health and sanity simply from fighting and Maxwell can easily regain lost sanity from his dapper personality. Glommer can also assist in lost sanity, although you may not want to bring her into the fight, lest she is killed by Deerclops.
Protection is also vital. Make a log suit and a football helmet. If playing as Wigfrid, make a battle helm instead of a football helmet as it is better. If you have enough marble, you could make a marble suit, but remember it will slow your character down.
If this is your first time facing off against Deerclops or if you are unconfident, find a way to revive yourself if you die. If it is your first year and you are playing the singleplayer game, the easiest thing to do is to is to find a touchstone. If you are playing in DST, hunt some spiders and make some telltale hearts if you are playing with friends. If you are playing in endless mode, you will not need to do any of this as you can simply revive at the Florid Postern. Put some chests filled with resources, tools, weapons, and armour near a touchstone or the Florid Postern if you are planning to revive with them to avoid running unarmed around the Deerclops trying to retrieve your stuff.
Deerclops will destroy your base and everything in it if you are there when it spawns, so journey off and make a war base. This base should contain as little as possible to avoid Deerclops destroying it. Make a fire pit, a Science Machine or Alchemy Engine if you really want one (though this is not recommended), and a chest if you have a lot of items. If you have it, put Glommer's Flower a safe distance from your war base, as you will need to regenerate your sanity after the fight. Don't place the flower too close to the base though, as Deerclops may kill Glommer before or during the fight. DO NOT place any ice boxes, crock pots, or farms as you will have prepared your foods before journeying to your war base (right?) and they will only be destroyed by Deerclops. If you are planning to revive with a touchstone or Florid Postern, place your base moderately close to it (but not too close in case Deerclops comes for you unprepared as you gather stuff from your chests).
Refer to the Deerclops page on the wiki for strategies for defeating Deerclops.