To start stacking, you need any of the basic bun hairs and the Silent Whisper bows. Note that the Fallen Feather suit hasn't returned on TW/CN since it debuted 2 years ago, so grab it while you can!
If you don't want to keep the bun hair in your hair stack, you start stacking by equipping the bun hair, then clicking on the bows twice. The bows will not show on Nikki and the item will not show as equipped - this is normal. Go back to the hair section and equip the hair you want to use to start stacking, and it will replace the bun hair. Then, click the bows again. This time they will show on Nikki, most likely broken and melted into the ground, but the item itself will not show as selected. This is fine. Select the hair you want to stack - this will not show as equipped or on Nikki. Go back and click the bows again. The broken part by her head should disappear, and your stacked hair will appear.
To stack another hair, select the bows again, then the hair you want to stack (invisible at this point), and then the bows again to make the stacked hair show up. You can keep going until you're satisfied or the game breaks.
If you want to keep the buns in your hair stack, during the start of the process click on the bows only once after selecting the bun, then proceed normally.
While layering different coloured hair can look very cool, if you want all your hair in the same colour so it looks more cohesive, you need to use all dyed hairs with the dyes activated. So, no awakened, collab or common items, i.e. hairs that can't be dyed normally, can be dyed through stacking. The colour of the last equipped hair is applied to all the other hairs in the stack.
This can break very easily (it broke for me when i made a stack and then did some dress stacking, any hair i added after that didn't change the colour of the previous stack) so it can help to have a plan starting out and using only solid dyes but it's also fun to experiment, especially with gradient hairs! The recent memory collection hair for example applies edge gradients on all the stacked hairs.
The only things that can't be stacked are hair ornaments, lenses and lipsticks. Dresses can be stacked with more dresses or even tops and bottoms, shoes can be stacked, headwear can be stacked, wings can be stacked (VIP Leonid wings look so cool with the original and awakened stacked...).
Just equip the hair -> select the bows twice -> equip a new hair -> equip a dress -> select the bows once -> equip another dress -> select the bows once, and continue to keep stacking more.
It works similarly for other categories. Handhelds especially benefit from this, because if you stack the same handheld it fixes permanently to Nikki, even on poses that don't usually allow handhelds.
Stacking foundations is a little different - for those, start the glitch (bun, bows twice, new hair) and then select the bows once and you can immediately start stacking foundations without needing to go back and forth with the bow. When you're done stacking, select the bows again so they're unequipped.
Be careful with this though, because every foundation stacked adds extra blush and Nail Salon reset doesn't always work with this.
The obvious way to fix stacked pieces not going away even after using delete or infecting your main screen set is to just close and reopen the game, but if your net sucks ass like mine and takes forever to reopen the game there's another way!
All you need to do is melt all the stacked items and then go to Nail Salon. Just click twice on all items that you've stacked, more times if necessary - just until the items break and collapse to the floor. Then go to Nails -> Nail Art to open the Nail Salon edit menu, then return to the dressing menu. Nikki should be free of the horrors! If you missed any parts last time they'll still be here, you can just repeat the melting and nail salon visit to fix those too.
It can be easier to identify stacked items by using the delete button first and then just melting whatever is still partially visible, but the delete button can sometimes not work and cause issues instead, so use at your own risk, especially if you've got a lot stacked. If all else fails, just close and reopen the game.
Note that doing the Nails method keeps the stacking glitch active - meaning you don't have to restart from the bun, you can just pick your base item, then bows, and stack from there.
Specifically for the bow melting into the ground, if you go to 3D Studio and back it will disappear.
Saving the glitched sets - it's not possible to save stacked items and it's probably best not to try.
Selecting the same hair twice - unless you want to melt it to reset the glitch, this is usually a bad idea and can break the dye stacking. Doesn't apply to different colours of the same hair, those give cool colour variations when stacked and are fun to try.
Returning to home screen without resetting the glitch - actually go for it. Everyone deserves to experience a little Nikki breaking.
Most things stacking I learned from people on the official discord server sharing tips and tricks (special thanks to LadyNaty, Adlers and Kami for the video guides). There's also this video and this doc that helped me get started. A Reddit user also shared this post of cool hair combos.
