I have made lots.
They actually are quite simple, because you are dealing with a square. It helps if you have a large space to cut out.
I would allow:
x2 for width. It is important to measure the window correctly. A good curtain rail goes past the edge of the window so that once the curtains are open they sit on the wall next to the window, not covering the edge of the window and cutting out light. So, measure the full width of the curtain pole.
x 2 for gather (unless you have very heavy fabric or very small/tight windows)
length. Look at the window, how long do you want them? Do you have rediators under the windows that shouldn't be covered? Are they bedroom windows and you are concerned about light? If you are concerned about light, they need to be a bit longer to make sure the light doesn't leak from the bottom.
I would allow a good 10 cm (4 inches) for the hem, as the weight makes them hang better.
lining - you can buy blackout lining by the metre, it is quite thick to work with. You need pretty much the same amount as the top fabric, but you don't need a big hem. The hem of the lining sits a couple of cm above the hem of the fabric, and it only needs a 1 cm hem.
Sides - the fiddly bit is the sides, you need the top fabric to come round the edge and fold over, so you make (eg) the curtain 1m 10 cm wide, and the lining 1 m wide. You sew the two sides together, and then when you put it flat the top fabric comes round onto the lining side 5 cm on each side. then you press a fold down the top fabric, and it is done.
You buy the heading tape by the metre. easy to sew on (straight and flat) Turn the top edge under so it catches under the heading tape. You can buy tape in different widths, long heavy curtains need a wider stronger tape. The tapes come with different gathers - straight gathering, or gathers into pleats.
When you have the measurements, add on 1 cm onto every edge to allow for the seams.