joeba has using the interface on a new computer down perfectly, but your still left with the problem of exporting it onto wowinterface, or whatever site you want to use.
If you download winrar (or other like it), you can make a subfolder (name it whatever you want to name your ui that you will post) and copy your interface and WTF folder into that subfolder.
from there, right-click the subfolder and click the "export and email" button, and once it has the .rar file ready to send, don't send it.
Instead, grab the .rar file from the attachments and move it to your subfolder.
then when you get onto wowinterface, just brwose and find the .rar file.
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there's probably FAR simpler ways than that to go about it, but it's the only way that i have found to do it lol
ALSO, make sure that you read through each addon page on wowinterface (or wherever you have downloaded the addon) and make sure you add any license agreements or copyright documents (.txt files) because you will get removed without proper documentation.
One last thing
, do a quick file search through your ui for wowmatrix.dat files and remove them, as your compilation will be removed for those as well.
Hope that helps