Measuring up a window is the easy part. Trish Ash writes for the person who has already bought the fabric and now has to decide whether the tabs get stitched into the top seam or buttoned on the right side, and whether the heading will be visible at all once a pelmet goes above it.
The curtain instructions go properly deep. She covers lined and unlined tab tops with buttons covered in a contrasting fabric, eyelet curtains made by the taped method, and why three inch eyelet tape carries heavier cloth than most people expect. A wooden pelmet box gets built from scratch, with a warning that fitting the hardware is heavy work and you will want somebody holding the other end.
There is a photographed run through the basic sewing tools and what each one actually does, which is the post to send anyone who owns a machine and very little else.
Around the sewing sit pieces on interior design colour, on keyword research, and on starting a home based service business, because the woman making curtains is often also wondering about being paid to make them. Pick your curtain style and work through that set of instructions.