I 've not seen any tutorials, but I'll tell you a bit of what I've learned through trial and error in the last couple of weeks.
Tiny clothing is all attachments. It's not like biggie clothing, you don't make it on the clothing layers, it has to be built from prims. I started by looking at a couple of free t-shirts, taking them apart and imitating them. Just work out what shape you need to make to cover the tiny body and go from there.
I found the best way was to make the basic prim for an item, get a rough size by rezzing it next to my AV and moving it to roughly where I wanted it. Then take it, attach it to where you want it to attach, and get the sizing/positioning just right. To add parts (buttons etc) you have to detach and rez because you cant add to the object while you are wearing it. I generally get the basic shape/size right, then add all the extra bits, then put it on and play around resizing/positioning parts through "edit linked prims". It's not quick, and the tiny will move slightly making it even more fiddly.
There are some problems with tiny clothes. Firstly because tiny bodies are all attachments as well, a lot of the attachment points may be used before you even get to put clothes on.
Secondly, because the actual AV body is deformed and crushed to get it to the right size, some of the attachment points aren't quite where you would expect them. In particular I've had trouble attaching things to hands. Fine when the AV is standing still, but as soon as you walk your attachment is wobbling around above your head. You must check the look of the clothes both standing still and moving around. You can get round these problems by using scripts I think, but that's beyond me.
Tiny bodies are proportioned very differently to biggie AVs. Short stubby arms and legs, and much thicker in the body. Sleeves and trouser legs will generally be too short to do much with them, and will often largely be hidden by the body anyway.
Finally remember that because all the tiny bodies are different sizes your clothes may not fit a different AV. I'm an otter, which is slimmer than a lot of other tiny AVs, ferrets are even slimmer than me, but hippos and elephants are much thicker in the body. If you are making to sell, then either you need different versions for different types of tiny, or you need to accept that your customers may have to modify clothes fairly extensively to get them to fit.
If you are used to the building tools it's not too hard to make stuff for tinies. The fitting can be a pain, but is really a case of trial and error and a lot of patience. I don't know about making things like flexi skirts, but I think you'd need to make them a bit stiffer than biggie ones, to avoid them drooping into the body. Most tiny skirts are not much more than frills anyway.
Hope that's useful, but I'm no expert and have just been playing at it for a couple of weeks. If you want a chat, IM me in world and we can experiment together

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