There is a lot to remember when obtaining all the trophies for this game in one run.
I prefer to play games in the souls series at my own pace and you can obtain the majority of trophies in this game whenever you want.
There is however, a number of missable objectives/items/NPC encounters that if missed can make the completion a lot longer than it needs to be.
This guide is not comprehensive and is entirely based on what is missable, I highly recommend reading the separate trophy guides for help on each the individual achievements. There are also many helpful guides elsewhere on the internet that prove quite helpful if your looking to make your character quite powerful
Don't defeat bed of chaos before speaking to Quelana of Izalith
You can find her in Blightown after getting a +10 pyromancy flame, she sells pyromancy's that you need to buy, she will give you a further pyromancy after you defeat the bed of chaos but you must speak to her before doing so
Don't defeat Gwyndolin before levelling her covenant
You need to obtain the darkmoon seance ring from the catacombs and level her covenant up once by giving her 10 souvenir of reprisals
Don't defeat Nito before levelling his covenant
Reaching Nito through a coffin in the catacombs and giving him 10 Eye of deaths is required for a miracle
Don't kill any NPC's
The only exception to this is Lautrec. He will drop a pretty useful ring and if left alive he will cause havoc in Firelink shrine, there is no trophy associated with keeping him alive but most NPC's are related in some way to an trophy so killing them is not recommended
Complete the NPC questlines for Big hat Logan, Siegmayer and Rhea
These are required for sorcery's and miracles, you need to save Big hat Logan in Sens Fortress and then in Duke's archives, Siegmayer in Sen's Fortress and in Anor Londo) and save Rhea in Tomb of the Giants
You only have once chance (3 for the gargoyles) per playthrough to get the tail weapons
Prioritise these during the boss fights, it won't be as easy on new game plus
Never use the boss souls for anything but the boss souls weapons
There are 15 boss souls weapons that need to be made in order to get the rare weapons achievement, the only boss souls you can use for souls are the DLC bosses
To get the Darkwraith covenant trophy you need to defeat the Four Kings straight after Ornstein and Smough without talking to the giant serpent in Firelink Shrine
Personally, I would recommend waiting until new game plus to obtain this as the Four Kings are quite difficult foes if your still a low level.
After saving Laurentius in the Depths, buy his pyromancies and get your pyromancy flame to +10 ASAP and be careful what you say to him
When he asks you about Blightown, make sure to say "No" as he may leave Firelink, becoming hollow and the spells he sells will be unobtainable until you start new game+
Buy Dusk of Oolaciles spells before obtaining the broken pendant
Not exactly missable but if you obtain the broken pendant in the Duke's archives after freeing her then her spells will now not be purchasable until you start the DLC. You only need to beat the first boss to encounter a mushroom called "Elizabeth" who will sell you these spells instead.
Note: The DLC is entirely optional and is not required for any achievement. It is however considered some of the best content in the game and there are some powerful spells available that can make the rest of the game a breeze. There is also a blue titanite slab in a much more user friendly location than the one in the main game.
You need to defeat every optional boss
Except the DLC and Taurus demon
Everything thing else in your first playthrough is unmissable. Before defeating the final boss, you should have all sorcery's and pyromancies. You should have every miracle except the "Sunlight Spear" which is obtained by giving Gwyns soul to the sunlight altar. You should also have every rare weapon except 8 boss soul weapons (You may also be missing the "Dark hand" if you planned on joining the darkwraith covenant on your 2nd playthrough)
You may want to get all the +10 weapons required for the boss soul weapons ready in this playthrough, so you don't have to obtain the large ember in the depths on your next new game
New Game +1
During the Ornstein and Smough fight, obtain the other boss soul to the one you got on your first playthrough
If you defeat Smough first you then face a more powerful Ornstein. When defeated you get Ornstein's soul. If this is what happened on your first playthrough then you need to do the opposite and defeat Ornstein first and then the more powerful Smough and vice versa
If you didn't on your first playthrough, remember to go straight to the Four Kings after getting the lordvessel
The only optional bosses you need to kill are the Moonlight butterfly and Gwyndolin
Choose the alternative ending to the one you chose on your first run
You can either link the fire and light the bonfire or you can walk outside the boss arena the way you came, simply choose the opposite to what you have already chosen.
Before killing Gwyn a 2nd time, you should only have 2 boss soul weapons left to complete
New Game +2
After killing Sif, ringing the 2 bells and making your way to Anor Londo via the Iron Golem, you need to run to the Giant Blacksmith to create the final 2 boss soul weapons and complete the game