The worms will often still move a lot when it is in a squeeze preparation (see Observation), and this can make it hard to make detailed observations. Worms can be anaesthetised with a solution of MgCl2 in distilled water (tap or mineral water will often also do). For marine samples I usually us a 7.14% solution (i.e. 7.14g per 100ml), and I place the worm into a 50:50 (or even 60:40) mixture of MgCl2-solution and sea water. In contrast, fresh-water species are much more sensitive to magnesium chloride, and so in these I will often use only a few percent of solution (too high concentration will stop the beating of the cilia, and make the epidermis crumply and less transparent, which is not nice for observation).
Once you have the worm in a nice squeeze preparation go through a series of magnification steps to document more and more detailed structures of the worm. I usually start taking pictures with the 4x objective, even if the worm is very small. This makes it easier later to reconstruct the magnification at which the different pictures were taken. I try to take pictures of the following structures: habitus, eyes, brain, pharynx, pharynx glands, testis, sperm in testis, ovary, forming eggs, eggs and/or sperm in female antrum, eggs, vaginal cilia, shell glands, false seminal vesicle, seminal vesicle, copulatory stylet, prostate glands, vesicula granulorum, male pore, male pore cilia, sperm, epidermis, epidermal cilia, sensory cilia, rhabdites, rhamites, adhesive glands, and food items.
For taxonomy the most important structure is of course the copulatory stylet, but also the structure of the female antrum and the sperm morphology is important. However, the sperm morphology is a little tricky to document, because you would like to document the ready sperm from the seminal vesicles, rather than the forming sperm form the testis. This means that you have to somehow make sure what you are looking at. 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 water (usually the stuff the worm was held in before you looked at it) next to the cover slip. The capillary force will draw in the water and the strength of the squeezing will be reduced. 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.
What I then usually try to achieve is to amputate the tail plate, and to make another squeezee preparation with this, while I'll place the frontal part of the worm into 100% Ethanol for a DNA sample. (Details on this to follow)