this is for a bare window with no wooden runners so bacically 6in above either side and below and that is the golden rule then when measuring fabric hang the curtain tape and measure from that when altering curtains always work from the top so cut of any axcess from threr and replace curtain tape, dont touch the bottomm at all.
if making up own curtains from fabric always work from bottom up that is pattern match from the bottom(because you are always going to be slightly out and the gather will hide it at the top.
when matching fabric always work in odd numbers ie 5 swags or at least 3 same with pattern dont go for 3 or 4 repeats big patterns need at least 5 repeats.always wash or dry clean your fabric before starting any venture because curtain fabric shrinks bt 3% consider a 100in drop thats a 3 inch loss (so you would end up with 97in drop) the good news is they will only shrink once and remember that a linning will often shrink at a differant rate than your outer shell,
dont be afraid to use dress fabrics here and there because it wears well and is alot cheaper,above all be confident its not the end of the world if you get it wrong.any questions please ask .
ps.always allow 9inches for hems a 4 and a 5 this will make the end product hang well and if it doesnt put 2pencees at the corners and joins,enjoy.