A lot of people make the mistake of thinking decluttering and organising are a "one and done" job but it isn't, they're a constant cycle of little and often to keep it that way.
Additionally not everyone organises the same way so it's important to make sure you're actually organising in a way you can keep it organised. There's no point having a Pinterest worthy cupboard with lots of little boxes organised alphabetically if you actually need it to be a more open and flexible system as you won't keep it that way. I am a visual macro organizer, as if it is out of sight then I will forget it exists and if there are too many steps to put something away I won't do it. DD on the other hand is a micro organiser who needs everything out of sight - so her stuff is organised into very specific categories and hidden away.
So, for example in the bathroom we have a sink unit with a cupboard on each side so we have one each. My cupboard has clear open acrylic boxes with categories like "make up" where all my make up is just in one big box and I go through it to find what I want, meanwhile DD has a little drawer unit in her cupboard where her make up is separated into different categories and each one has its own drawer... She couldn't cope with the visual mess of my big box of mixed stuff but I couldn't keep up with having to put everything in a specific drawer each time so we are both organised but in different ways and neither way is wrong, they just work for different people.
When decluttering/organising initially it can seem massively overwhelming so that puts a lot of people off but you can do it in small chunks here and there. Personally I do a mix of "progress not perfection" organising - where I set a timer or have a set time limit eg an ad break in a TV show, waiting for the kettle to boil or the microwave to ping etc and when that time is up then I just stop wherever I'm up to - and intentional organisation - where I set aside time to do a job in its entirety.
Personally when decluttering and organising I try to do it in stages to make things easier. I choose one area - whether it's a whole room or just a drawer - and focus on that exclusively. Unless the rest of the house is burning down I am not bothered if there's clothes all over DDs floor or a pile of post sitting on the counter or the sofa cushions are a mess, my focus is on the one drawer/shelf/cupboard I'm sorting.
Firstly I make a quick initial rubbish sweep - just go through the area and throw away anything that is literal rubbish, whether that's half opened packaging that has been shoved in the drawer, sweet wrappers, dried up pens, broken things, random items that you don't need or want etc, don't think about it, just pick up an item and if it's rubbish, bin it and if it isn't then put it back.
After the rubbish sweep, or sometimes at the same time if it's a small area, I go through and take out anything that I want to keep but doesn't belong in that space and put it in a "crap box" to be dealt with later. I have a small crap box in all the main areas of my home and anything that doesn't live in that room that needs to be returned to its home can be put in there throughout the day and then at the end of the night every week or so I will take 10 mins to return everything to it's rightful place.
Once you've got rid of the rubbish and stuff in the wrong place then you'll often be surprised how few items there actually are to organise. Then you go through and decide if there's anything you want to get rid of and stick that in a box to donate/recycle (or if it's too overwhelming to think about having to donate or sell things, especially in an initial organisation session, then feel free to bin it). Personally I always have a box for donations in my bedroom and in a Kallax unit in my hall so that as soon as I find something to donate (whether it's a top I don't wear, something DD has outgrown or just a whisk I don't use) I just drop it in the box and then when the box is full or it's been a couple of months then I will take it to charity. That way I got into the habit of regular donations, I get the item out of its original home so I can make space without having to deal with it immediately and there is a designated home for the item so it isn't cluttering up my space.
Sometimes I will just do one part of the cycle - either look for rubbish, re-home items, look for things to donate/sell - depending on the time I have available and the inclination I have to do so because I'd rather do a little bit than nothing at all as it's still some sort of progress.
I am a natural hoarder so I really struggle to part with things, especially if they might be useful in future, or if I buy a new X but the old one is still functional then I would often keep the old one "just in case" but now I am really strict about having a 'one in, one out' policy. When I want to buy something I make myself decide what item it is going to be replacing and then get rid of that item first (unless it's a purchase where I need to know I have the new item before disposing of the old one) so that I'm not tempted to buy the new one and then keep the old one anyway.
To supplement that I also set physical limits and if there is more of something than will fit in the designated storage then I have to purge it until it does. Eg I have a drawer for kitchen tools - I can have as many spatulas as I want as long as they all fit in the drawer, however if the drawer no longer shuts I have to decide what things to get rid of to make it shut ... That might mean I have 10 spatulas and no whisk or it might mean I sacrifice a couple of spatulas to make room for the whisk but I get to determine what quantities work for me rather than deciding "I only actually need one spatula at a time so I must throw out the other 9 to make room for the whisk, even though I use all of the spatulas on a regular basis for different things but only use a whisk once a year and without it I could just use a fork if necessary".
As part of that I don't feel bad about getting rid of part of a set of I know I'm not going to use it. In the past I have held on to items that I know I am not going to use but because they are part of a set I felt obliged to keep them - eg hoover attachments I knew I would never use or measuring cups where I only use 3 out of the 7 in the set. Now I have no issue with seeing that in a set of XYZ I only use X and Z so I can throw away Y rather than trying to find space to keep it just because it's part of the XYZ set. It's surprisingly liberating to be able to get rid of those items.
When it comes to getting rid of stuff that might be useful in the future looking at you, random box of cords I haven't needed for years but cannot throw away I justify whether to keep it based on the question: if I get rid of this today and then need it next week, how difficult is it going to be to source a new one? If the answer is it will be a 5 minute job to get a new one and/or will cost pennies to replace the item then I get rid of it. If the answer is it will be really difficult/a lot of effort to replace then I can either keep it indefinitely or set myself a time frame and if I haven't used it in that time frame then I get rid of it anyway as I do not need it enough to justify keeping it.