I'll answet this in numbers (basically going down the list)..
1. Do you have a pattern already? If not I will tell you..Literally make a tube with one end a little wider than the other.. Like this:
The top and bottom would have lines but I can;t make those right now..But that's the idea. Not as wide at the bottom, but I think you get the point.
For the elastic, make the part that goes around your upper leg smaller than your leg (meaning it has to stretch to fit around your leg) and that;s the top. For the bottom to get that poof look, do the exact same thing around your ankle. You can make it a little wider than your ankle to give it a great look.
2. That's great!! I'd do that for my Rin if I had the money lol But would you mind linking to the tutorial please and thank you? I'd love to see.
3. It's just a typical male school fuku thing. If you look up the girls summer uniforms you will get a basic idea..If you have a shirt around the same size you need, and it fits without having to stretch around your body when it's on (so a semi loose fitting shirt) just trace it..
4. They are just black shorts..If you can't find shorts in the right length or a good material, buy some slacks and cut them into shorts then hem them.
5. Look up "vocaloid headphone tutorial" they are much easier than they look..
6. This one will be a little hard..You will either need to buy premade ones on like ebay or something, or find something with the right shape if you are an accuracy buff. If you don't think you can, I suggest the ebay route..You'd search for "Len Kagamine Shoes"
7. The collar itself? I basically gave you the info needed in the shirt talk, but they are super simple.
By the way one last thing..Remember that not everyone knows characters, so try to post reference pictures when asking about things. Also one other thing, you only said his name, and not what version. Being a HUGE Rin and Len fan I know what you are asking, but other's might not so knowing which version would help..Anyway hope I helped
Cosplay DA Regular DA
Kumoricon 2014 (so far):
-Sora (Kingdom Hearts 2)
-Serena (Pokemon Y [It's the one I have lol])
-Akaito (Vocaloid. Black & Red Ver.)