Dunelm Mill do ready made black out linings. I gave a spare pair to my daughter earlier this week, they fit a 56 drop 74 width and cost £8.50 so I wonder if it'd cost more to make them.
Ana - I remember that sense of achievement from making curtains but I don't think it's something I'm drawn to currently. I'm having some blackout heavier ones made for the bedroom. Good old Dunelm have some of the same fabrics as John Lewis at half the cost, plus they do a make and measure service in the same way that JL do. They do not (sadly) have the Arts and Crafts/William Morris fabrics that JL sell but I'm prepared to compromise 