Sustainable fashion doesn't mean buying "sustainable" clothes and donating "non-sustainable" clothes... Rather, what we already have in our closets are the most sustainable items we can wear!
Most people think that donating clothes in a responsible, ethical, and sustainable way to get rid of unwanted items, but "donations" are simply a rebrand for waste colonialism. There is no "away" on planet Earth, just more of an "out of sight, out of mind" sort of situation... So the first step is to just consume less (or not at all if possible!) But most people already have a lot of stuff even without anything new, but instead of going all Marie Kondo on your closet here are some alternatives to donating clothes that you can't use/don't want:
Hoarding is totally an option! If you have the space, try to hold on to your clothes, especially synthetics. The items that you keep won't go to a landfill, and they don't have to just sit in your closet either... Below are some creative ideas on how to transform those seemingly "unusable" items into some of your favorite pieces!
Typically people don't use certain clothes because they don't fit... For clothes that are too big consider having them tailored... Certain items can also be sized up, specifically things like men's dress pants. Not everything can be resized, but there are other ways to make use out of clothes that no longer fit (especially if you still have an attachment to them!)
If there is a hole or stain or other issue, clothing and accessories can also be repaired/mended... For something like a hole, you can try this kit. If you clothes need a different type of mending, check out this subreddit for some mending inspiration. If something is stained you can try removing it yourself or take it to a dry cleaner (make sure they are away of the stain). Also, try to remove stains as soon as possible and DO NOT DRY items as it will set the stain; if the fabric is natural, you can also consider dyeing it to cover stubborn stains (however, thread is usually at least partially synthetic so you cannot change the color of the thread by dyeing it). Or if something is not worth the effort, they can always become work clothes.
And finally, you can consider upcycling your item somehow... There are many resources for upcycling in Pad 3: The Power of Community, which you can access by signing up for Module 3 (or by signing up for all Modules). It's best to try and upcycle your item without the use of a lot of chemicals or other inputs, but remember that the end result is that you love and can wear the item again... So get creative! Some ideas for upcycling garments into other functional items (that aren't clothes) include making a quilt, tote, pots for plants, furniture, soft sculpture/art, rags, dog rope toys, trivets, etc. And if you're not feeling inspired, you can also commission a designer such as Janelle Abbott to upcycle your items for you.
This can either be through a swap event or directly with an individual... This is a great way to get something new without purchasing a new item, and by getting rid of your clothes in this manner you should be able to find someone who genuinely wants what you're giving away/swapping, which means that your item(s) will actually be used. You may be able to find people through community such as the subreddit.
Items that are in decent condition and of good quality should be sold online or consigned... This can help you make some money back on your items, and whoever buys your item will most likely give it a second (or third) life because they see its value. Selling online will give you more control, but consigning your items will make it easier on you.
And when you purchase items, remember that quality items will not only be enjoyed by you while you own them but that they can also be resold, giving you some cash back and giving the garment another life after you are done with it (but you should really be buying stuff that you can use for your entire lifetime, but I realize that people change and evolve).
So this is still a donation, but it's different than what most people think of... Most people are giving to Goodwill or The Salvation Army, but these operations receive more donations than they can process and clothes inevitably get dumped... Similarly, items given to fast fashion donation schemes typically don't make it to the people that need them (see first link in this blog post), but all of this can be avoided by giving locally.
Local vintage shops/charities don't receive as many donations as national operations, meaning that they'll have more time and a better shot at getting picked up by someone... Charities that address homelessness are a good example of an organization that will have a real need for your unwanted garments... So don't just go with the biggest operation if you want your clothes to actually be used and valued by someone else.
