You need a miter box or miter saw. Mark the 45 on the quarter round as if it is in the corner. Then just cut and install. There is nothing fancy about cutting quarter round. You only run into problems with crown moulding. The reason for this is because the moulding does not sit in a corner, and it usually has some sort of decrative profile.
For help on crown moulding, see:
http://www.josephfusco.org/Articles/Crown_Moulding/crownscript.html
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