From soil you can extract the nematodes by modified Cobb's decanting and sieving followed by modified Baermann's funnel techniques. From plants, if the nematodes are in the plant tissue e.g. roots you should wash and cut the roots into small pieces and boil in lactophenol for about five minutes in the presence of some stain like cotton blue. Then you can find the Nematodes present in the roots under a stereoscopic microscope and take out the same by dissecting the roots with the help of needles. If there is still more information required ... I can give more details.