In order to work efficiently with extracted worms you, first and foremost, need a high quality compound microscope. Ideally it should have interference contrast (DIC or Nomarski) and if possible also phase contrast (good for visualising sensory hairs and cilia, and also sperm cells). The microscope should have a range of objectives (e.g. 4x, 10x, 20x, 40x, and 100x oil immersion), to visualise details at different scales. These creatures are very small (usually only about 1mm long), so high magnification is essential to reveal the detailed anatomy.
Making a good squeeze preparation is essential to immobilise the worms and to visualize the various internal structures, especially in the bigger species (see pictures). I generally use plasticine (or children's modeling clay) to make small feet on the corners on a cover slip (my favourite size is 21x26 mm). Then I place the cover slip down on a drop containing a worm on a microscope slide, and gently press down the corners. Be sure to put enough water initially, so that the worm does not get squeezed too much by the capillary force on the cover slip. Then use a corner of a filter paper to remove excess water, while you observe the worm in a binocular (preferably under transmitted light). Once the worms is lightly squeezed you can place it under the compound microscope and start observing and documenting the worm.
making a squeeze preparation
Go through a series of magnification steps to document more and more detailed structures of the worm (maybe the attached sample and documentation sheets will give you some ideas what structures you should look for).
With a bit of practise you will be able to recover worms from the squeeze preparations if you want to do some other things with it. To do this place a drop of the water the worm came from next to the cover slip. The capillary force will draw in the water and the strength squeezing of the squeezing will be relaxed. Generally the worm will start moving around again below the cover slip and you can then lift the cover slip with pair of fine watchmakers forceps.