How to make a window seat cushion?

A rectangular cushion wouldn't be hugely difficult but piping can be a bit fiddly and takes a little bit of practice to get on nicely. If you had a bit of experience you could pretty much make your own pattern up to fit, but if you have no experience of sewing you'd probably be better getting a book and following the instructions carefully - as long as you take it slowly and steadily and don't cut corners when you don't know what you're doing you'd probably be fine.

Sewing and soft furnishing books are the kind of things that are very often available in charity shops, fetes and second-hand shops so it wouldn't cost much, but I think a book is better for a beginner than a webpage because it will have extra sections explaining terminology and techniques and just give you more depth of information.

As far as the feathers go... Foam would be far, far easier for this sort of thing, because it will hold the square shape of the cushion, whereas feathers will be much floppier and saggier, so you would need to put quite a bit of stiffening into the structure of the cushion cover and probably make an under-cushion with some sort of quilting or something to hold the feathers into the right shape. If you are completely set on feathers then I would recommend starting by searching for a premade cushion inner of the right size and shape - somewhere like dunelm have quite a range. Or perhaps you could get the single piece of foam as the base and create some kind of quilted undercushion cover with the feathers that the foam would slot into to give the outside the slightly softer feathery look. Then make a cushion to go round the whole lot?

It might be more difficult as it's not a straightforward rectangle.
And I think dander's suggestion of foam and feathers combined sounds good. A lot of fabric shops and markets will be able to cut the foam to size - I doubt they would have what you want as standard.
Have a look at your local library too, might be able to get a book or even dvd for instructions. Both Laura Ashley & Readers Digest had good 'how to make home furnishings......' books but may be out of print now.