I've sort of managed this (it's still sort of a work in progress). I think you need to be a bit brutal. If you have the time, try things on one by one. If it doesn't fit (and is unlikely to ever fit, rather than being a £100 dress that's a little loose or tight) or is poor quality, or you don't really like it - Vinted/donate/chuck. A lot of clothes I donated last year were Boohoo circa 2012 - bodycon dresses, polyester bodysuits! I'm just never going to wear that again because I'm not 21 and clubbing.
Worth noting that Vinted can be a lot of effort for not much return. Unless your clothes are branded or high end you're not likely to get much for them. I ended up reducing lots to £1 or £2 and still not shifting it. I think kids clothes are more popular, especially bundles.
I gradually improved and replaced my wardrobe with more timeless quality pieces. It was naturally quite simple as I have a bigger budget now than 15 years ago, so I do have a bit more to spend. There are some Youtubers and Instagrammers who can be quite good for this. You need to find someone whose style you like and would suit you and is wearable. I really love Lydia Tomlinson but she wears so much white and beige and kitten heels. I use her videos for inspiration, but I live in a wet and muddy part of the country with a black spaniel so... It's just not realistic.
Shops to look at - H&M, Mango, Cos, &Other Stories, M&S, Zara, Abercrombie, although they can all be hit and miss. I do try to stick to natural fabrics in neutral or monochrome colours. It makes getting dressed so much easier as you can mix and match separates. Get a few good quality jackets - a leather/bomber jacket and a trench coat would be good for spring. As the seasons change, add a different coat. You don't need a new coat every season (how I was raised, buy cheap, buy multiple times!), instead I've built up a collection of woollen coats, waterproof puffers, light jackets etc that work for every occasion. When buying things, DO NOT keep anything you sort of like but aren't sure you'll wear. Only keep something if you can see yourself wearing it as part of a minimum of three outfits, and it suits your lifestyle. I.e. don't keep a gorgeous pair of heeled boots if you have a 2 year old and a dog and what you really need is trainers.