Hi Andy, SWMBO here.
The technique you will use depends on the effect you want to create. If you just want moveable box cushions with foam innards you need to sew a box cushion cover (ie a cubical container for the cushion) piped along the edges for strength/appearance, with a zip on the bottom face so that you can remove the cover for washing - great for when the spag bol gets tipped on it.... For this, you ideally need 4 - 5" foam, obtainable from any upholstery shop.
If you want a fixed back, attached to a ply/chipboard sheet, the technique is somewhat different. You need to shape the foam in order to accommodate the bottom cushion and so that the top is sloped. I suggest you first glue - yes glue (copydex is ideal) the foam onto the backing and use a large knife to shape the foam. Remember to cut the foam about 2" larger on the top edge than you need: fold the foam over the top of the backing to create a soft-look finish. Glue the foam over the top then get your fabric. Starting at the bottom, staple (high tech stuff here!) the material squarely to the back of the board, allowing about 2" of material as turn back. Always start in the middle and work out to the edges.
Once that is done, smooth the material up over the foam and pull down, quite firmly. Remember that the material will stretch/foam will condense with time so it really is important to pull and smooth till you've got a firm backing. Staple the back, starting from the middle again and pulling and smoothing the front to ensure a good finish. Stop about 2" from the side and do the same with the sides. Then turn the raw edges at the corners in to create a neat finish - practise till you get the desired effect (a 'hospital corner' always looks smart...). Staple these and, if you want to go the whole hog, sew the two edges together.
Remember that if you're doing the solid backing version, you will need to shape the foam on the seat to accommodate the back cushion. Also, if you're turning a corner with the back, you'll need to shape the foam in that corner so that it looks neat.
This is very much the 'cheats' way of doing this. You could go as far as to make a frame, use webbing, horsehair and kapok, sewing loops to fill with horsehair and strengthening the edges using an upholsterers knot, but I guess that'll keep for another day.....!
If you want some good guides, there are several 'beginner's guides' out there, but a course (evening classes) is your best bet if you really want to get into it all. If any of the above is unclear, I can expand on details...
(edit - Shady's comment: and we thought our hobby was full of technical jargon, tricks and tips.... :shock: )