Firstly, you need to establish what the coil springs are sitting on.
Coil springs sit inside a frame on top of either jute webbing, metal straps, wooden rails or onto a flat wooden base. The webbing is on the underside of the frame, making it possible to re-web a chair without removing the springs, padding and fabric.
Next, arrange the springs ontop of the webbing and sew them into place with jute twine and a strong needle. You only need 3-4 stitches to hold each spring in place.
The top of the springs are compressed and tied together with jute twine, &/or compressed down using a layer of hessian. If using hessian, you will need to sew the springs to the hessian with a large curved needle and strong twine, just like you did earlier.
Once the springs are sewn on the bottom and the top, the springs form a comfortable platform for the rest of your stuffings to sit on.
The height of the spring has many different factors, but here is a general rule of thumb -- measure the height of the side frame and add approx 10cm. For instance, if the frame is 8cm high, buy a spring about 18-20cm.
How much should I compress the spring down?
Press the spring down with your hand. When you feel the spring starting to resist, that is the height to tie it off at. Basically, you want to compress it enough to give you enough resistance on the top padding and fabric, but not so much that it starts to force the bottom out.